\section{Traversal phase --- proofs}
For each operation called for by the traversal algorithms, we prove
that the commit generation preconditions are met.
\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.