\end{enumerate}
-After processing each $\pc$ we will have created:
-
-\begin{itemize}
-
-\item $\tipcn$ and $\tipcy$ such that $\baseof{\tipcy} = \tipcn$.
-
-\end{itemize}
+After processing each $\pc$ we will have created $\tipcn$ and $\tipcy$
+such that:
+
+\statement{Correct Base}{
+ \baseof{\tipcy} = \tipcn
+}
+\statement{Base Ends Supreme}{
+ \tipcn \ge \pendsof{\allsrcs}{\pcn}
+}
+\statement{Tip Ends Supreme}{
+ \tipcy \ge \pendsof{\allsrcs}{\pcy}
+}
\subsection{$\alg{Merge-Base}(\pc)$}
\subsection{Tip Base Merge}
+$L = W$, $R = \tipcn$.
+
+TODO TBD
+
+Afterwards, $\baseof{W} = \tipcn$.
+
+\subsection{Tip Source Merge}
+
+In fact, we do this backwards: $L = S$, $R = W$. Since $S \in \pcy$,
+the resulting $C \in \pcy$ and the remaining properties of the Merge
+commit construction are symmetrical in $L$ and $R$ so this is fine.
+
+By the results of Tip Base Merge, $\baseof{W} = \tipcn$.
+
+By Base Ends Supreme, $\tipcn \ge \baseof{S}$ i.e.
+$\baseof{R} \ge \baseof{L}$.
+
+Either $\baseof{L} = \baseof{M}$, or we must choose a different $M$ in
+which case $M = \baseof{S}$ will suffice.
+