Definition: Given the Question and Answer pair, detect whether the answer is acceptable or not. Denote acceptable answer as "yes", otherwise denote as "no". An acceptable answer is consistent with the context in question, detailed, long, and uses general knowledge of English. Answers that are not definite and suggest a variety of options are not acceptable answers.
Input: Question: As generally agreed and as extensively discussed in this question, "an" should be used in place of the more common "a" where the following word begins with a vowel sound. I have just encountered for the first time an instance of the phrasing an mule in the book Why Chemical Reactions Happen (James Keeler and Peter Wothers, Oxford University Press, 2003). (This book is not mine. I was sitting opposite a friend when the friend mentioned the unusual phrasing.) This phrasing appears to contradict the rule discussed above, since "mule" is, as far as I am aware, pronounced /mjul/ (see Wiktionary). Here is the context. The author is using an analogy to explain a chemistry concept (the italics are not mine, but the bold emphasis is): A useful analogy here is that of an mule, which we get by crossing a horse with a donkey. To describe an mule as a horse or a donkey would be inaccurate and it certainly does not interconvert rapidly between the two! Rather we need to recognize that although an mule has parts that are reminiscent of a horse and parts that a reminiscent of a donkey, it is something altogether different. Looking for other examples online, I have managed to find one on Wikipedia's List of Siege engines. Discussing a Siege engine, the Scorpio: Similar to the ballista, but smaller. Was sometimes mounted on an mule-drawn cart. Other examples are difficult to find, but here's one from a user-posted question on Answers.com (referring, notably, to a kind of deer rather than the animal, a mule): Is an mule deer an omnivore? No. Mule deer are herbivores. ... What is going on? Is there an irregular pronunication of "mule" of which I am not aware? Is this some strange language quirk, some archaic spelling that I am encountering for the first time? 
Answer: It's just a typo, and it probably originally said "an ass". That would have been changed because it's incorrect (an ass is a donkey, not a horsedonkey cross) and, even if it were correct, the fact that ass means well, you know, ass, might be distracting enough to make it worth changing. Another source of this kind of typo is when an adjective has been added or removed and the article hasn't been updated (e.g., "an elephant" becoming "an big elephant" or "an amazing coincidence" becoming "an coincidence"). That's presumably not happened in this particular text, since there are several instances of "an mule".

no