-When we are trying to do a merge of some kind, in general,
-we want to merge some source commits $S_0 \ldots S_n$.
-We'll write $S_0 = L$. We require that $L$ is the current git ref
-for $\patchof{L}$.
+When we are trying to do an update of some kind, in general,
+for each patch $\pc$
+we want to merge some source commits $S \in \set S_{\pc}$.
+We require $\patchof{S} = \pc$,
+and $\tipcc \in \set S_{\pc}$.