Merge Ends applies.
-$D \isin Y \equiv D \le Y$. $D \not\isin X$.
-
-Consider $D = C$.
+$D \isin Y \equiv D \le Y$. $D \not\isin X$. Recall that we
+are considering $D \in \py$.
+
+Consider $D = C$. Thus $C \in \py, L \in \py$.
+But $X \not\haspatch \p$ means xxx wip
+But $X \not\haspatch \p$ means $D \not\in X$,
+
+so we have $L = Y, R =
+X$. Thus $R \not\haspatch \p$ and by Tip Self Inpatch $R \not\in
+\py$. Thus by Tip Merge $R \in \pn$ and $M = \baseof{L}$.
+So by Base Acyclic, $M \nothaspatch \py$. Thus we are expecting
+$C \haspatch \py$. And indeed $D \isin C$ and $D \le C$. OK.
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