Y \haspatch \p
\implies
\begin{cases}
- M \haspatch \p : & \displaystyle
- \bigforall_{E \in \pendsof{Y}{\py}} E \le M \\
+ M \haspatch \p : & \pendsof{Y}{\py} = \pendsof{M}{\py}
+ \\
M \nothaspatch \p : & \displaystyle
\bigforall_{E \in \pendsof{X}{\py}} E \le Y
\end{cases}
Consider $D \neq C, M \haspatch P, D \isin Y$:
$D \le Y$ so $D \in \pancsof{Y}{\py}$ so by Merge Ends
-and Transitive Ancestors $D \le M$.
+and Transitive Ancestors $D \in \pancsof{M}{\py}$ so $D \le M$.
Thus $D \isin M$. By $\merge$, $D \not\isin C$. OK.
Consider $D \neq C, M \haspatch P, D \not\isin Y$: