merge any Topbloke-controlled branch into any plain git branch.
Given those conditions, Tip Merge and Merge Acyclic do not apply.
-And by Foreign Contents for Y, $\forall_{\p, D \in \py} D \not\le Y$
+And by Foreign Contents for (wlog) Y, $\forall_{\p, D \in \py} D \not\le Y$
so then by No Replay $D \not\isin Y$
so $\neg [ Y \haspatch \p ]$ so neither
Merge Ends condition applies.