Meaning of $\uparrow$ in below d-separation algorithm from Koller In Probabilistic Graphical Models by Koller and Friedman there is an algorithm to find the nodes reachable from node $X$ via trails that are active, given conditioning set $Z$. What is the meaning of "$\uparrow$" at line 22, in the algorithm given below (given in image format, due to its length)? -

Does "trail up through $Y$" denote configuration "$\rightarrow Y \rightarrow$"?

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*$\uparrow$ denotes travelling from children to their respective parents. $(M,\uparrow)$ denotes that $M$ is approached from its children - "... $\leftarrow M$".

*$\downarrow$ denotes travelling from parents to their respective children. $(M,\downarrow)$ denotes that $M$ is approached from its parents - "... $\rightarrow M$".

We can see how this convention satisfies our notions of a trail being active.
Case I - $(M,\uparrow)$
The trail goes from a child node to $M$ and may extend from $M$

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*either to its children, as in $\leftarrow M \rightarrow Y$, - "$(Y,\downarrow)$"

*or its parents, as in $\leftarrow M \leftarrow Y$. - "$(Y,\uparrow)$"

If $M$ is not observed, i.e., $M \notin Z$, both trails till $Y$ remain active. This is captured in lines 23-26.
Case II - $(M,\downarrow)$
The trail goes from a parent node to $M$ and may extend from $M$

*

*either to its children, as in $\rightarrow M \rightarrow Y$, - "$(Y,\downarrow)$"

*or its parents, as in $\rightarrow M \leftarrow Y$. - "$(Y,\uparrow)$"

If $M$ is not observed, $\rightarrow M \rightarrow Y$ remains active. If $M$ or its descendants are observed, i.e., $M \in A$, $\rightarrow M \leftarrow Y$ remains active. This is captured in lines 28-34.
