Answers.Body: <p>This is half of a twin problem. The other half has white knight d8. See <a href="https://pdb.dieschwalbe.de/P0000829" rel="nofollow noreferrer">link in PDB</a>. Good, in my opinion, to keep the integrity of a problem unless proposing an explicit version.</p>
  <p>Black's last move was c7-c5 or c6-c5.</p>
  <p>In a) wSd8, we can eliminate c6-c5, so ep is on, but White has lost castling rights. So the only combination of rights possible is <strong>S_ep</strong>. Hence 1. bxc6ep#!</p>
  <p>In b) wRd8 shown here, both c7-c5 &amp; c6-c5 remain a possibility. Castling is possible, but only if the last move was c6-c5. So 3 combinations of rights remain: <strong>S_castling</strong>, <strong>S_ep</strong> &amp; <strong>S_neither</strong>, but not <strong>S_both</strong>. If we apply the conventions, we may castle but not ep. Hence 1. 0-0-0#!</p>
  <p>But what is the name for the protocol used in b)?</p>
  <p>Let's take a step back. RS (Retro Strategy) &amp; PRA (Partial Retro Analysis) are the two main alternative protocols to help us resolve multiple interpretations of conditional moves. They are defined <a href="https://chess.stackexchange.com/questions/29971/what-are-the-castling-e-p-conventions-and-how-are-they-used">here</a>.</p>
  <p>As someone who's spent a lot of time doing pure mathematics, I like the idea that our definitions of RS &amp; PRA should scale all the way down to situations where we only have a single interpretation, after applying the castling &amp; e.p. conventions.</p>
  <p><a href="https://i.sstatic.net/L3lm3ldr.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/L3lm3ldr.png" alt="scaled down" /></a></p>
  <p>For example, if there are no conditional moves, then the situation is like in the lefthand box in the picture above. There is just one interpretation. RS &amp; PRA will behave the same when applied to it. The same is true if retroanalysis has eliminated all uncertainty about the conditional moves, as in twin a) of Keym's problem.</p>
  <p>If we have a single conditional move, then things aren't much more complicated. See the middle &amp; righthand boxes in the picture above. If the conditional move is castling, then the castling convention tells us that by default we are allowed to castle. And if the conditional move is ep, then the ep convention tells us that by default we <em>cannot</em> castle. So either way, there is a single green box representing the interpretation which we follow. The yellow boxes represent combos that are globally dominated due to the castling/ep conventions, and we don't need to consider them.</p>
  <p>In all three of these simple cases, RS &amp; PRA will behave the same as one another. Now this may all seem very trivial, so let's turn to twin b) of Keym's problem.</p>
  <p><a href="https://i.sstatic.net/1iAF0U3L.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/1iAF0U3L.png" alt="two diamonds" /></a></p>
  <p>The lefthand diamond above is the relevant one. As can be seen, there is still only one green box. Due to the conventions, we never get to ep, but we may castle. Because there are no choices to be made, RS &amp; PRA behave identically to one another, just like they did in the earlier trivial cases.</p>
  <p>So do we say that this is RS or PRA? Or both? Or neither? That is a matter of taste, but I hope the general message gets across:</p>
  <blockquote>
  <p><strong>Just one green box =&gt; RS &amp; PRA behave the same</strong></p>
  </blockquote>
  <p>For complex cases, of course RS &amp; PRA behave very differently to one another. But I enjoy the fact that they agree in simpler cases.</p>
  <h2>Special PRA (SPRA) &amp; Special RS (SRS)</h2>
  <p>It's instructive to compare with SPRA (Special PRA). SPRA is defined to mean that ep is optimistic. So the ep convention is different: there is no change at the protocol level. It's hence misleading to call it SPRA. And we might just as well have SRS for &quot;Special Retro Strategy&quot;.</p>
  <p>Now looking at the righthand diamond above, it's clear there are two green boxes. While there is no <em>significant</em> difference in the behaviour between SRS &amp; SPRA here, they are distinguishable. SRS gives two solutions, as we can play either of the optimistic moves to checkmate. SPRA on the other hand has a single solution with two parts, and in each of them one of the mating moves is permitted.</p>
  <p>It's obvious that optimistic ep is not intended here. The twinning between a) &amp; b) obviously asks that the solution in one is the retro try in the other. So normal pessimistic ep is the answer.</p>
  <h2>Retro Variants (RV)</h2>
  <p>While not condemning RV, it's a term whose meaning has shifted, and is generally deprecated. Rather than reusing an old term with a new meaning (argh!) as Werner Keym seems to propose. I would rather use a new acronym ARV for &quot;All Retro Variants&quot;. This indicates the meaning, and avoids confusion with previous usages.</p>
  <p>So basically ARV means that the castling &amp; ep conventions are suspended. All the arrows are gone in the boxes below. All the yellow boxes become green. We <em>still</em> have the choice between RS &amp; PRA protocols for ARV, although I suspect that existing ARV problems are designed for PRA.</p>
  <p>In this particular problem, the solutions to twin b) are a mess with ARV, and do not harmonize with a) as intended.</p>
  <h2>Wrap-Up</h2>
  <p>I hope this is a useful worked exercise, to help clarify a complicated space. The main lessons for me are:</p>
  <ol>
  <li>Conventions and protocols are very different things. We need both.</li>
  <li>The two main protocols, RS &amp; PRA, are indistiguishable in very simple cases.</li>
  <li>SPRA uses a different convention, but the protocol remains PRA.</li>
  <li>Similarly, ARV throws away the conventions, and we are still free to choose the protocol.</li>
  <li>When analyzing a problem from the past, we should look at what the author was trying to achieve (in so far as we can determine this), and this will help us best to communicate it to a new audience.</li>
  </ol>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2024-07-04T04:19:56.750Z
Answers.Text: In the original, yes, a twin. On d8, there is a knight instead of a rook. With the knight, the case is unambiguous - only one last move of black is possible. And there are no questions "What is it?".
Answers.UserDisplayName: Элси Ринген
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Answers.Text: There is a good article with examples: https://www.janko.at/Retros/Glossary/Castling-and-En-passant-Jun-cm.htm
  It is clear that the problem is being solved, as RS. It's more interesting why it's not a PRA. Please note - only because of the strict limitations of 16.2.
  Pay attention to task 15. There is a similar case. There is an RS solution. But the task is marked either RV (the options are determined by the last move of black) or SPRA (e.p. by default). And two answers.
  The same could be done with this task.
Answers.UserDisplayName: Элси Ринген
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Answers.CreationDate: 2024-07-04T04:51:28.230Z
Answers.Text: In general, it turns out like this. Without marking, the problem is solved as RS. PRA does not work because of the very rigid position of paragraph 16.2. But if desired, you can mark RV or SPRA to the problem and make two solutions in the problem. RV and SPRA will give the same answer - there is only a difference in the division approach used.
Answers.UserDisplayName: Элси Ринген
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Answers.Text: @ЭлсиРинген You keep saying "paragraph 16.2 [of the Codex, about e.p.] is rigid". I do think it's badly written, and I interpret it in my own way as "if we don't know whether last move was double hop, then ep is not permitted". But what is this rigidity that troubles you?
Answers.UserDisplayName: Laska
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Answers.Text: I was referring to this particular case. Black's last move is either c6-c5 (has the right to castling) or c7-c5 (has the right to e.p.). That is, we always have one of the two rights. And they are interdependent and almost mutually exclusive. Almost only because of rule 16.2.
Answers.UserDisplayName: Элси Ринген
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Answers.CreationDate: 2024-07-04T07:34:45.837Z
Answers.Text: Because 16.2 will not skip the option (there is no castling - there is e.p.). Although the only reason for the absence of castling for white is the move c7-c5, rule 16.2 will not consider this sufficient proof (they say, the last move could have been c6-c5, and white does not have castling for another reason). Despite the fact that the PRA Convention does not imply the deprivation of any rights without grounds.
Answers.UserDisplayName: Элси Ринген
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Answers.Text: That is, in fact, there was a pure case of PRA (two rights that are interdependent and mutually exclusive, given that no one can be deprived of any right without sufficient grounds), which rule 16.2 did not approve for completely formal reasons. Because 16.2 found something to find fault with.
Answers.UserDisplayName: Элси Ринген
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Answers.Text: And in general, pay attention to the fact that if there is at least one reason why you can prohibit e.p., rule 16.2 always does it.
  As in another task, you persuaded 16.2 to skip e.p. based on a vague assumption IF I do not know. :)
Answers.UserDisplayName: Элси Ринген
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Answers.Text: I really don't understand your general difficulty with 16.2. The Codex is written loosely, and although 16.2 says "An en-passant capture on the first move is permitted only if it can be proved that the last move was the double step of the pawn which is to be captured." I read it as "If it cannot be proved whether an ep is legal, it is not permitted." Is that any better for you. Castling & ep conventions are dual to one another - neither should give us more grief than the other. Are you ok with that?
Answers.UserDisplayName: Laska
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Answers.Text: RV is a really outdated term. But before, in fact, he designated all the tasks that can be divided into parts (and the most common approach of dividing into parts was according to black's last move). RV then designated both modern PRA and SPRA tasks, as well as tasks with the task "Result?".
Answers.UserDisplayName: Элси Ринген
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Answers.Text: Now it makes sense to use it if we are talking at all about the possibility of solving the problem by dividing into parts. Here, in a narrower sense, by dividing the task into parts according to Black's last move. In 2010, Keym offered an RV for such cases. In 2011, the SPRA. The solution is the same in both cases - just different ways to bypass rule 16.2. I personally prefer the RV method (according to black's last move).
Answers.UserDisplayName: Элси Ринген
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Answers.Text: Laska, I am satisfied with your answer. And I have no difficulty with rule 16.2. I understand that the Rule always requires irrefutable evidence.
Answers.UserDisplayName: Элси Ринген
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Answers.Text: There is no need to change anything specifically in this task. Here is a twin with two different solutions. I have given the problem simply as an example of this kind of problem, where the author can, at his discretion, make it solvable in one way (RS) or in two versions (SPRA, RV).
Answers.UserDisplayName: Элси Ринген
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Answers.Text: There is an obvious error in the response. How did I not notice right away? e.p. and castling are interdependent and incompatible. They are impossible at the same time.
Answers.UserDisplayName: Элси Ринген
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Answers.Text: @Elsie, your last comment is correct. In b) ep & castling care not both possible. I've fixed my answer thank you. RS & PRA still both have 1 solution, while under SRS & SPRA there are 2 solutions & 1 solution 2 parts, respectively. Does RV add anything? I think it used to mean all retro combos (3 here) but is now deprecated.
Answers.UserDisplayName: Laska
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Answers.CreationDate: 2024-07-07T18:48:20.423Z
Answers.Text: No, Andrew. The PRA in this task was simply canceled. Only the RS solution.
  Or put notes so that there is a partial solution.
Answers.UserDisplayName: Элси Ринген
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Answers.CreationDate: 2024-07-07T18:53:58.527Z
Answers.Text: RV may be outdated. But task 15 (from Keim's article) was solved for 55 years until 2008 by analyzing Black's last moves. And now we can't even identify this method in any way.
Answers.UserDisplayName: Элси Ринген
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Answers.CreationDate: 2024-07-07T18:57:09.563Z
Answers.Text: And until 2008, this was generally the only way to solve this problem.
Answers.UserDisplayName: Элси Ринген
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Answers.CreationDate: 2024-07-07T19:06:11.693Z
Answers.Text: And what is the result? The task was transferred to the SPRA. I doubt very much that Fabel would have been satisfied with this decision.
Answers.UserDisplayName: Элси Ринген
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Answers.CreationDate: 2024-07-08T08:23:16.183Z
Answers.Text: Check out beautiful new pictures. Don't worry about 2008 change of default in stipulation. It doesn't matter
Answers.UserDisplayName: Laska
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Answers.CreationDate: 2024-07-09T14:34:04.993Z
Answers.Text: "SRS gives two solutions"
  That's not so. Either of the two solutions will be a complete solution to the problem (since the second part is excluded), the two together will give one complete two-part solution. The formula is 1+1=1.
Answers.UserDisplayName: Элси Ринген
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Answers.Text: I agree that RV is not a good fit for the "last move method" due to the inevitable confusion with the old meaning. But the method itself is good because it does not change or modify anything in paragraphs 16.1 and 16.2. The method (until 2008) has been used for more than 50 years, very easy for an ordinary user to understand. And now it's just left without a name. The easiest thing here is to give it a name.
  And that's the easiest way to call it. "The last move method" (LMM or LM).
Answers.UserDisplayName: Элси Ринген
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Answers.UserDisplayName: Laska
Answers.Body: <pre><code>[Title &quot;#1 - W. Keym. Die Schwalbe, 1969. Ded. K. Fabel&quot;]
  [FEN &quot;qrbR4/rP1P1pp1/pQ1pp3/pPp5/6P1/1P3BB1/3PP2P/R3K1k1 w - - 0 1&quot;]
  </code></pre>
  <p>==</p>
  <pre><code>[Title &quot;#1 (RV or SPRA)  - Karl Fabel. problem 1953&quot;]
  [FEN &quot;8/1B4p1/8/3Pp3/7b/4P3/1P1PPQ2/R3K2k w - - 0 1&quot;]
  </code></pre>
  <p>The first task is a part of gemini. There were considerations for choosing a mark. The second part was interesting for our question. Just in case, I brought a second similar task.</p>
  <p>Both tasks have the same structure: incompatibility of e.p. and castling. In the first one, there is a rather serious preliminary retroanalysis.</p>
  <p>Until 2008, the problem was solved without marking by the RS method (one solution), marked RV - two partial solutions. Moreover, RV could mean any method: PRA, SPRA, according to the last possible moves of black. The solution is the same for any method.</p>
  <p>After 2008, the PRA method became the default method, and RS software is now applied secondarily if a PRA solution is not possible. Therefore, without marking, we first need to understand if there is a PRA here.</p>
  <p>The PRA Convention is explicitly described in the Codex.</p>
  <p><strong>Partial Retrograde Analysis (PRA) convention. Where the rights to castle and/or to capture en-passant are mutually dependent, the solution consists of several mutually exclusive parts. All possible combinations of move rights, taking into account the castling convention and the en-passant convention, form these mutually dependent parts. If in the case of mutual dependency of castling rights a solution is not possible according to the PRA convention, then the Retro-Strategy (RS) convention should be applied: whichever castling is executed first is deemed to be permissible.</strong></p>
  <p>The initial position can have three states: S_castling, S_ep, S_neither.
  The PRA Convention, guided by paragraph 16.1, will exclude S_neither from consideration (there is no reason for white's lack of castling). And we got two interdependent rights (one excludes the possibility of the second) and two mutually exclusive parts. Everything is fine here!
  But it was not possible to come to an agreement with paragraph 16.2. Unlike the PRA, rule 16.2 considers that the case of S_neither may be in the starting position (for which, of course, there is no solution). And therefore, we will not be able to apply PRA in this task. And by default, it will be solved by the RS method. Moreover, it is necessary to bring at least a few last moves:
  <strong>1. ... c6-c5 2. Qc7-b6 b6xNa5</strong> 3. Be4-f3 Kg2-g1</p>
  <p>==</p>
  <p><strong>If the author wants (as in the second problem) that the problem has two partial solutions</strong>, you need to put a mark. But how?
  In the well-known article by Werner Keym in the 2010 version of the article, the problem was labeled RV, and the solution was as follows:</p>
  <p>.1. ... c7-c5 1. bxc6ep#</p>
  <p>.1. ... any other move by black (exd5, Be8-h5, Kg1-h1) 2. 0-0-0#</p>
  <p>But in the 2011 version of the article, the mark was already SPRA and the solution, respectively:</p>
  <p>.1. bxc6ep#;</p>
  <p>.1. 0-0-0#</p>
  <p>Personally, I like the RV method much more. Determining options based on Black's last move is much easier for a simple user than delving into the logic of PRA, for example, and there is no straightforwardness, as in SPRA (where paragraph 16.2 was simply taken and canceled).</p>
  <p>But there is a problem. The RV name will cause confusion with the old RV and, probably, it is better to come up with some other name for the &quot;black's last move&quot; method. By the way, this will also be useful for tasks like Results.</p>
  <p>==</p>
  <p>UPD. About &quot;do not disturb the dead.&quot; Andrew condemns the use of the outdated term RV. Only it wasn't my idea.</p>
  <p><strong>Many retro problems which were published with the supplementary stipulation “Retro Variants” or “RV” before 2008 are PRA problems after the modification of the codex 2008 and need no supplement any more. Yet the term “Retro Variants” is not superfluous. I propose to use this term, which is not used any more in the codex, when all possible last moves and their effects on the solution are searched. Because of this new definition “Retro Variants” clearly differ from the PRA convention. The latter deals with the combination of special move rights, the former with all possible last moves.</strong></p>
  <p>This is a direct quote from Werner Keym (2010).</p>
  
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Answers.CreationDate: 2024-07-08T09:12:52.140Z
Answers.Text: (1) I have made changes to my answer, including RV at your request. Please review and respond (in a focused but informative way). (2) It's important in SE to keep focus on the original questions. If you want to solve the Fabel problem, put it in a new question, and I will respond there. (3) Fabel is genuine SPRA, Keym is genuine RS/PRA, so there is no real connection, although both are #1. Thanks
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Answers.Text: (1) I agree that RS and PRA use the same facts in their constructions. But I don't agree that they behave the same way. They can't behave the same way because they have different goals from the beginning.
  The separation into parts in both methods follows the same principle: the S_neither state is considered impossible by default.
  But after that, they already behave differently. RS excludes one part and gives a solution for only one part, PRA wants to give a solution for each part.
Answers.UserDisplayName: Элси Ринген
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Answers.CreationDate: 2024-07-09T14:16:53.960Z
Answers.Text: As a result, there is no solution to PRA in the problem (due to the fact that rule 16.2 considers the state of S_neither possible, but there is no solution for it). And the task can only be solved by RS.
Answers.UserDisplayName: Элси Ринген
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Answers.UserDisplayName: Элси Ринген
Body: <pre><code>[Title &quot;#1 - W. Keym. Die Schwalbe, 1969. Ded. K. Fabel&quot;]
  [FEN &quot;qrbR4/rP1P1pp1/pQ1pp3/pPp5/6P1/1P3BB1/3PP2P/R3K1k1 w - - 0 1&quot;]
  </code></pre>
  <p>The solution to the problem is simple. Black has only two possible last moves: c7-c5 and c6-c5.</p>
  <p>But which is the correct retro protocol to apply?</p>
  <p><strong>RS spoke first: &quot;just do it 1. 0-0-0#&quot;</strong></p>
  <p><strong>PRA: &quot;We see two solutions here. 1. 0-0-0# (not e.p.); 1. bxc6ep# (not 0-0-0). But 16.2 will say that this is not our case.&quot;</strong></p>
  <p><strong>RV: &quot;Why not ours? Very much ours. 0. ... c7-c5 1. bxc6# (not 0-0-0); 0. ... c6-c5 1. 0-0-0#&quot;</strong></p>
  <p>Which of the three of them gave the correct answer? Or maybe it’s a SPRA instead?</p>
  <p><strong>Background</strong>: problems like this need a protocol to solve them. The two main protocols are PRA (Partial Retro Analysis) &amp; RS (Retro Strategy), explained <a href="https://chess.stackexchange.com/questions/29971/what-are-the-castling-e-p-conventions-and-how-are-they-used">here</a>.</p>
  <p>There are also two rarer protocols, RV (Retro Variants) &amp; SPRA (Special PRA)? All four are discussed in an authoritative article <a href="https://www.janko.at/Retros/Glossary/Castling-and-En-passant-Jun-cm.htm" rel="nofollow noreferrer">here</a> by IM Werner Keym who authored the relevant sections of the World Federation for Chess Composition Codex (and who composed the above problem).</p>
  
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2024-07-03T21:16:36.820Z
Comments.Text: What is a **RS**?  **PRA**?  **RV**?  **SPRA**?  Recommend the asker [edit] the question to include definitions of these acronyms to help future visitors.
Comments.UserDisplayName: SecretAgentMan
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2024-07-03T21:59:03.050Z
Comments.Text: Respectfully, I was only trying to help improve the question so it can attract more upvotes over time, and attract high-quality answers.  I find your questions very interesting. The value of posts is much bigger than just an answer to the asker--it is for future visitors on the site.
Comments.UserDisplayName: SecretAgentMan
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2024-07-03T22:01:33.793Z
Comments.Text: To be clear, no one is telling you what to do as a _Vsevobuch_. My [comment](https://chess.stackexchange.com/questions/45846/what-is-the-answer-to-this-problem-what-should-be-applied-here-pra-rs-rv#comment74796_45846) clearly uses the word _recommend_.
Comments.UserDisplayName: SecretAgentMan
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2024-07-04T09:33:36.077Z
Comments.Text: Coming from the HNQ (the title is currently shown on almost all SE sites), curious what are PRA, RS, and RV in chess, never heard of them until now.
Comments.UserDisplayName: Andrew T.
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Comments.CreationDate: 2024-07-04T09:59:10.170Z
Comments.Text: @ЭлсиРинген People may very well be able to help you or be interested in the problem even without knowing some acronyms in English. What's the problem with explaining what they are?
Comments.UserDisplayName: JiK
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2024-07-04T17:45:54.573Z
Comments.Text: Voting to reopen this interesting question.  The asker should consider editing the question to make it more understandable for future visitors (recommend [edit] feature).
Comments.UserDisplayName: SecretAgentMan
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2024-07-04T21:16:34.160Z
Comments.Text: Have retitled and expanded the acronyms and given links to detailed explanations of this very technical but genuine question. Suggest it can be reopened
Comments.UserDisplayName: Laska
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2024-07-07T15:16:42.787Z
Comments.Text: Andrew T. PRA, RS и прочее, это просто методы. Зачастую мы их используем в задачах, даже не зная, как они называются. Это скорее теоретический вопрос, нежели практический.
Comments.UserDisplayName: Элси Ринген
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2024-07-07T16:28:23.917Z
Comments.Text: Nils Lindemann
  I didn't understand anything that was written here. Anyway, I don't care.
Comments.UserDisplayName: Элси Ринген
Tags: <retrograde-analysis>
Title: What is the correct retro protocol for this problem? PRA or RS?
ThreadText: # What is the correct retro protocol for this problem? PRA or RS?
  
  (Asked by: username_0 on 2024-07-03)
  
  ```
  [Title "#1 - W. Keym. Die Schwalbe, 1969. Ded. K. Fabel"]
  [FEN "qrbR4/rP1P1pp1/pQ1pp3/pPp5/6P1/1P3BB1/3PP2P/R3K1k1 w - - 0 1"]
  
  ```
  
  The solution to the problem is simple. Black has only two possible last moves: c7-c5 and c6-c5.
  
  But which is the correct retro protocol to apply?
  
  **RS spoke first: "just do it 1. 0-0-0#"**
  
  **PRA: "We see two solutions here. 1. 0-0-0# (not e.p.); 1. bxc6ep# (not 0-0-0). But 16.2 will say that this is not our case."**
  
  **RV: "Why not ours? Very much ours. 0. ... c7-c5 1. bxc6# (not 0-0-0); 0. ... c6-c5 1. 0-0-0#"**
  
  Which of the three of them gave the correct answer? Or maybe it’s a SPRA instead?
  
  **Background**: problems like this need a protocol to solve them. The two main protocols are PRA (Partial Retro Analysis) & RS (Retro Strategy), explained [here](https://chess.stackexchange.com/questions/29971/what-are-the-castling-e-p-conventions-and-how-are-they-used).
  
  There are also two rarer protocols, RV (Retro Variants) & SPRA (Special PRA)? All four are discussed in an authoritative article [here](https://www.janko.at/Retros/Glossary/Castling-and-En-passant-Jun-cm.htm) by IM Werner Keym who authored the relevant sections of the World Federation for Chess Composition Codex (and who composed the above problem).
  
  **Tags:** retrograde-analysis
  
  > **Comments:**
  > **username_1**: What is a **RS**?  **PRA**?  **RV**?  **SPRA**?  Recommend the asker [edit] the question to include definitions of these acronyms to help future visitors.  
  > **username_1**: Respectfully, I was only trying to help improve the question so it can attract more upvotes over time, and attract high-quality answers.  I find your questions very interesting. The value of posts is much bigger than just an answer to the asker--it is for future visitors on the site.  
  > **username_1**: To be clear, no one is telling you what to do as a _Vsevobuch_. My [comment](https://chess.stackexchange.com/questions/45846/what-is-the-answer-to-this-problem-what-should-be-applied-here-pra-rs-rv#comment74796_45846) clearly uses the word _recommend_.  
  > **username_2**: Coming from the HNQ (the title is currently shown on almost all SE sites), curious what are PRA, RS, and RV in chess, never heard of them until now.  
  > **username_3**: @username_0 People may very well be able to help you or be interested in the problem even without knowing some acronyms in English. What's the problem with explaining what they are?  
  > **username_1**: Voting to reopen this interesting question.  The asker should consider editing the question to make it more understandable for future visitors (recommend [edit] feature).  
  > **username_4**: Have retitled and expanded the acronyms and given links to detailed explanations of this very technical but genuine question. Suggest it can be reopened  
  > **username_0**: username_2 T. PRA, RS и прочее, это просто методы. Зачастую мы их используем в задачах, даже не зная, как они называются. Это скорее теоретический вопрос, нежели практический.  
  > **username_0**: Nils Lindemann I didn't understand anything that was written here. Anyway, I don't care.  
  
  ---
  
  ## 2 answers
  
  ### Answer by username_4 on 2024-07-03. Score: 4
  
  This is half of a twin problem. The other half has white knight d8. See [link in PDB](https://pdb.dieschwalbe.de/P0000829). Good, in my opinion, to keep the integrity of a problem unless proposing an explicit version.
  
  Black's last move was c7-c5 or c6-c5.
  
  In a) wSd8, we can eliminate c6-c5, so ep is on, but White has lost castling rights. So the only combination of rights possible is **S\_ep**. Hence 1. bxc6ep#!
  
  In b) wRd8 shown here, both c7-c5 & c6-c5 remain a possibility. Castling is possible, but only if the last move was c6-c5. So 3 combinations of rights remain: **S\_castling**, **S\_ep** & **S\_neither**, but not **S\_both**. If we apply the conventions, we may castle but not ep. Hence 1. 0-0-0#!
  
  But what is the name for the protocol used in b)?
  
  Let's take a step back. RS (Retro Strategy) & PRA (Partial Retro Analysis) are the two main alternative protocols to help us resolve multiple interpretations of conditional moves. They are defined [here](https://chess.stackexchange.com/questions/29971/what-are-the-castling-e-p-conventions-and-how-are-they-used).
  
  As someone who's spent a lot of time doing pure mathematics, I like the idea that our definitions of RS & PRA should scale all the way down to situations where we only have a single interpretation, after applying the castling & e.p. conventions.
  
  [![scaled down](https://i.sstatic.net/L3lm3ldr.png)](https://i.sstatic.net/L3lm3ldr.png)
  
  For example, if there are no conditional moves, then the situation is like in the lefthand box in the picture above. There is just one interpretation. RS & PRA will behave the same when applied to it. The same is true if retroanalysis has eliminated all uncertainty about the conditional moves, as in twin a) of Keym's problem.
  
  If we have a single conditional move, then things aren't much more complicated. See the middle & righthand boxes in the picture above. If the conditional move is castling, then the castling convention tells us that by default we are allowed to castle. And if the conditional move is ep, then the ep convention tells us that by default we *cannot* castle. So either way, there is a single green box representing the interpretation which we follow. The yellow boxes represent combos that are globally dominated due to the castling/ep conventions, and we don't need to consider them.
  
  In all three of these simple cases, RS & PRA will behave the same as one another. Now this may all seem very trivial, so let's turn to twin b) of Keym's problem.
  
  [![two diamonds](https://i.sstatic.net/1iAF0U3L.png)](https://i.sstatic.net/1iAF0U3L.png)
  
  The lefthand diamond above is the relevant one. As can be seen, there is still only one green box. Due to the conventions, we never get to ep, but we may castle. Because there are no choices to be made, RS & PRA behave identically to one another, just like they did in the earlier trivial cases.
  
  So do we say that this is RS or PRA? Or both? Or neither? That is a matter of taste, but I hope the general message gets across:
  
  > **Just one green box => RS & PRA behave the same**
  
  For complex cases, of course RS & PRA behave very differently to one another. But I enjoy the fact that they agree in simpler cases.
  
  ## Special PRA (SPRA) & Special RS (SRS)
  
  It's instructive to compare with SPRA (Special PRA). SPRA is defined to mean that ep is optimistic. So the ep convention is different: there is no change at the protocol level. It's hence misleading to call it SPRA. And we might just as well have SRS for "Special Retro Strategy".
  
  Now looking at the righthand diamond above, it's clear there are two green boxes. While there is no *significant* difference in the behaviour between SRS & SPRA here, they are distinguishable. SRS gives two solutions, as we can play either of the optimistic moves to checkmate. SPRA on the other hand has a single solution with two parts, and in each of them one of the mating moves is permitted.
  
  It's obvious that optimistic ep is not intended here. The twinning between a) & b) obviously asks that the solution in one is the retro try in the other. So normal pessimistic ep is the answer.
  
  ## Retro Variants (RV)
  
  While not condemning RV, it's a term whose meaning has shifted, and is generally deprecated. Rather than reusing an old term with a new meaning (argh!) as Werner Keym seems to propose. I would rather use a new acronym ARV for "All Retro Variants". This indicates the meaning, and avoids confusion with previous usages.
  
  So basically ARV means that the castling & ep conventions are suspended. All the arrows are gone in the boxes below. All the yellow boxes become green. We *still* have the choice between RS & PRA protocols for ARV, although I suspect that existing ARV problems are designed for PRA.
  
  In this particular problem, the solutions to twin b) are a mess with ARV, and do not harmonize with a) as intended.
  
  ## Wrap-Up
  
  I hope this is a useful worked exercise, to help clarify a complicated space. The main lessons for me are:
  
  1. Conventions and protocols are very different things. We need both.
  2. The two main protocols, RS & PRA, are indistiguishable in very simple cases.
  3. SPRA uses a different convention, but the protocol remains PRA.
  4. Similarly, ARV throws away the conventions, and we are still free to choose the protocol.
  5. When analyzing a problem from the past, we should look at what the author was trying to achieve (in so far as we can determine this), and this will help us best to communicate it to a new audience.
  
  > **Comments:**
  > **username_0**: In the original, yes, a twin. On d8, there is a knight instead of a rook. With the knight, the case is unambiguous - only one last move of black is possible. And there are no questions "What is it?".  
  > **username_0**: There is a good article with examples: https://www.janko.at/Retros/Glossary/Castling-and-En-passant-Jun-cm.htm It is clear that the problem is being solved, as RS. It's more interesting why it's not a PRA. Please note - only because of the strict limitations of 16.2. Pay attention to task 15. There is a similar case. There is an RS solution. But the task is marked either RV (the options are determined by the last move of black) or SPRA (e.p. by default). And two answers. The same could be done with this task.  
  > **username_0**: In general, it turns out like this. Without marking, the problem is solved as RS. PRA does not work because of the very rigid position of paragraph 16.2. But if desired, you can mark RV or SPRA to the problem and make two solutions in the problem. RV and SPRA will give the same answer - there is only a difference in the division approach used.  
  > **username_4**: @username_0 You keep saying "paragraph 16.2 [of the Codex, about e.p.] is rigid". I do think it's badly written, and I interpret it in my own way as "if we don't know whether last move was double hop, then ep is not permitted". But what is this rigidity that troubles you?  
  > **username_0**: I was referring to this particular case. Black's last move is either c6-c5 (has the right to castling) or c7-c5 (has the right to e.p.). That is, we always have one of the two rights. And they are interdependent and almost mutually exclusive. Almost only because of rule 16.2.  
  > **username_0**: Because 16.2 will not skip the option (there is no castling - there is e.p.). Although the only reason for the absence of castling for white is the move c7-c5, rule 16.2 will not consider this sufficient proof (they say, the last move could have been c6-c5, and white does not have castling for another reason). Despite the fact that the PRA Convention does not imply the deprivation of any rights without grounds.  
  > **username_0**: That is, in fact, there was a pure case of PRA (two rights that are interdependent and mutually exclusive, given that no one can be deprived of any right without sufficient grounds), which rule 16.2 did not approve for completely formal reasons. Because 16.2 found something to find fault with.  
  > **username_0**: And in general, pay attention to the fact that if there is at least one reason why you can prohibit e.p., rule 16.2 always does it. As in another task, you persuaded 16.2 to skip e.p. based on a vague assumption IF I do not know. :)  
  > **username_4**: I really don't understand your general difficulty with 16.2. The Codex is written loosely, and although 16.2 says "An en-passant capture on the first move is permitted only if it can be proved that the last move was the double step of the pawn which is to be captured." I read it as "If it cannot be proved whether an ep is legal, it is not permitted." Is that any better for you. Castling & ep conventions are dual to one another - neither should give us more grief than the other. Are you ok with that?  
  > **username_0**: RV is a really outdated term. But before, in fact, he designated all the tasks that can be divided into parts (and the most common approach of dividing into parts was according to black's last move). RV then designated both modern PRA and SPRA tasks, as well as tasks with the task "Result?".  
  > **username_0**: Now it makes sense to use it if we are talking at all about the possibility of solving the problem by dividing into parts. Here, in a narrower sense, by dividing the task into parts according to Black's last move. In 2010, Keym offered an RV for such cases. In 2011, the SPRA. The solution is the same in both cases - just different ways to bypass rule 16.2. I personally prefer the RV method (according to black's last move).  
  > **username_0**: username_4, I am satisfied with your answer. And I have no difficulty with rule 16.2. I understand that the Rule always requires irrefutable evidence.  
  > **username_0**: There is no need to change anything specifically in this task. Here is a twin with two different solutions. I have given the problem simply as an example of this kind of problem, where the author can, at his discretion, make it solvable in one way (RS) or in two versions (SPRA, RV).  
  > **username_0**: There is an obvious error in the response. How did I not notice right away? e.p. and castling are interdependent and incompatible. They are impossible at the same time.  
  > **username_4**: @Elsie, your last comment is correct. In b) ep & castling care not both possible. I've fixed my answer thank you. RS & PRA still both have 1 solution, while under SRS & SPRA there are 2 solutions & 1 solution 2 parts, respectively. Does RV add anything? I think it used to mean all retro combos (3 here) but is now deprecated.  
  > **username_0**: No, username_2. The PRA in this task was simply canceled. Only the RS solution. Or put notes so that there is a partial solution.  
  > **username_0**: RV may be outdated. But task 15 (from Keim's article) was solved for 55 years until 2008 by analyzing Black's last moves. And now we can't even identify this method in any way.  
  > **username_0**: And until 2008, this was generally the only way to solve this problem.  
  > **username_0**: And what is the result? The task was transferred to the SPRA. I doubt very much that Fabel would have been satisfied with this decision.  
  > **username_4**: Check out beautiful new pictures. Don't worry about 2008 change of default in stipulation. It doesn't matter  
  > **username_0**: "SRS gives two solutions" That's not so. Either of the two solutions will be a complete solution to the problem (since the second part is excluded), the two together will give one complete two-part solution. The formula is 1+1=1.  
  > **username_0**: I agree that RV is not a good fit for the "last move method" due to the inevitable confusion with the old meaning. But the method itself is good because it does not change or modify anything in paragraphs 16.1 and 16.2. The method (until 2008) has been used for more than 50 years, very easy for an ordinary user to understand. And now it's just left without a name. The easiest thing here is to give it a name. And that's the easiest way to call it. "The last move method" (LMM or LM).  
  
  ---
  
  ### Answer by username_0 on 2024-07-07. Score: 1
  
  ```
  [Title "#1 - W. Keym. Die Schwalbe, 1969. Ded. K. Fabel"]
  [FEN "qrbR4/rP1P1pp1/pQ1pp3/pPp5/6P1/1P3BB1/3PP2P/R3K1k1 w - - 0 1"]
  
  ```
  
  ==
  
  ```
  [Title "#1 (RV or SPRA)  - Karl Fabel. problem 1953"]
  [FEN "8/1B4p1/8/3Pp3/7b/4P3/1P1PPQ2/R3K2k w - - 0 1"]
  
  ```
  
  The first task is a part of gemini. There were considerations for choosing a mark. The second part was interesting for our question. Just in case, I brought a second similar task.
  
  Both tasks have the same structure: incompatibility of e.p. and castling. In the first one, there is a rather serious preliminary retroanalysis.
  
  Until 2008, the problem was solved without marking by the RS method (one solution), marked RV - two partial solutions. Moreover, RV could mean any method: PRA, SPRA, according to the last possible moves of black. The solution is the same for any method.
  
  After 2008, the PRA method became the default method, and RS software is now applied secondarily if a PRA solution is not possible. Therefore, without marking, we first need to understand if there is a PRA here.
  
  The PRA Convention is explicitly described in the Codex.
  
  **Partial Retrograde Analysis (PRA) convention. Where the rights to castle and/or to capture en-passant are mutually dependent, the solution consists of several mutually exclusive parts. All possible combinations of move rights, taking into account the castling convention and the en-passant convention, form these mutually dependent parts. If in the case of mutual dependency of castling rights a solution is not possible according to the PRA convention, then the Retro-Strategy (RS) convention should be applied: whichever castling is executed first is deemed to be permissible.**
  
  The initial position can have three states: S\_castling, S\_ep, S\_neither.
  The PRA Convention, guided by paragraph 16.1, will exclude S\_neither from consideration (there is no reason for white's lack of castling). And we got two interdependent rights (one excludes the possibility of the second) and two mutually exclusive parts. Everything is fine here!
  But it was not possible to come to an agreement with paragraph 16.2. Unlike the PRA, rule 16.2 considers that the case of S\_neither may be in the starting position (for which, of course, there is no solution). And therefore, we will not be able to apply PRA in this task. And by default, it will be solved by the RS method. Moreover, it is necessary to bring at least a few last moves:
  **1. ... c6-c5 2. Qc7-b6 b6xNa5** 3. Be4-f3 Kg2-g1
  
  ==
  
  **If the author wants (as in the second problem) that the problem has two partial solutions**, you need to put a mark. But how?
  In the well-known article by Werner Keym in the 2010 version of the article, the problem was labeled RV, and the solution was as follows:
  
  .1. ... c7-c5 1. bxc6ep#
  
  .1. ... any other move by black (exd5, Be8-h5, Kg1-h1) 2. 0-0-0#
  
  But in the 2011 version of the article, the mark was already SPRA and the solution, respectively:
  
  .1. bxc6ep#;
  
  .1. 0-0-0#
  
  Personally, I like the RV method much more. Determining options based on Black's last move is much easier for a simple user than delving into the logic of PRA, for example, and there is no straightforwardness, as in SPRA (where paragraph 16.2 was simply taken and canceled).
  
  But there is a problem. The RV name will cause confusion with the old RV and, probably, it is better to come up with some other name for the "black's last move" method. By the way, this will also be useful for tasks like Results.
  
  ==
  
  UPD. About "do not disturb the dead." username_2 condemns the use of the outdated term RV. Only it wasn't my idea.
  
  **Many retro problems which were published with the supplementary stipulation “Retro Variants” or “RV” before 2008 are PRA problems after the modification of the codex 2008 and need no supplement any more. Yet the term “Retro Variants” is not superfluous. I propose to use this term, which is not used any more in the codex, when all possible last moves and their effects on the solution are searched. Because of this new definition “Retro Variants” clearly differ from the PRA convention. The latter deals with the combination of special move rights, the former with all possible last moves.**
  
  This is a direct quote from Werner Keym (2010).
  
  > **Comments:**
  > **username_4**: (1) I have made changes to my answer, including RV at your request. Please review and respond (in a focused but informative way). (2) It's important in SE to keep focus on the original questions. If you want to solve the Fabel problem, put it in a new question, and I will respond there. (3) Fabel is genuine SPRA, Keym is genuine RS/PRA, so there is no real connection, although both are #1. Thanks  
  > **username_0**: (1) I agree that RS and PRA use the same facts in their constructions. But I don't agree that they behave the same way. They can't behave the same way because they have different goals from the beginning. The separation into parts in both methods follows the same principle: the S_neither state is considered impossible by default. But after that, they already behave differently. RS excludes one part and gives a solution for only one part, PRA wants to give a solution for each part.  
  > **username_0**: As a result, there is no solution to PRA in the problem (due to the fact that rule 16.2 considers the state of S_neither possible, but there is no solution for it). And the task can only be solved by RS.  
  
  ---
  
  