If $D = C$, trivial. For $D \neq C$:
$D \isin C \equiv D \isin A \equiv D \le A \equiv D \le C$. $\qed$
+\subsection{Foreign Contents:}
+
+Only relevant if $\patchof{C} = \bot$. Trivial by Foreign Contents of
+$A$. $\qed$
+
+xxx fixme not trivial use Totally Foreign Contents
+
\section{Create Base}
Given $L$, create a Topbloke base branch initial commit $B$.
$\qed$
+\subsection{Foreign Contents:}
+
+Not applicable. $\qed$
+
\section{Merge}
Merge commits $L$ and $R$ using merge base $M$:
$\qed$
+\subsection{Foreign Contents:}
+
+xxx use Totally Foreign Contents
+
+If $\patchof{C} = \bot$, by Foreign Merges
+$\patchof{L} = \patchof{R} = \bot$.
+
\end{document}