+\section{Ranking phase}
+
+We run the following algorithm:
+\begin{enumerate}
+\item Set $\allpatches = \{ \}$.
+\item Repeatedly:
+\begin{enumerate}
+\item Clear out the graph $\hasdirdep$ so it has no edges.
+\item Execute {\bf Rank-Recurse}($\pc_0$)
+\item Until $\allpatches$ remains unchanged.
+\end{enumerate}
+\end{enumerate}
+
+{\bf Rank-Recurse}($\pc$) is:
+\begin{enumerate}
+
+\item If we have already done {\bf Rank-Recurse}($\pc$) in this
+ranking iteration, do nothing. Otherwise:
+
+\item Add $\pc$ to $\allpatches$ if it is not there already.
+
+\item Set
+$$
+ \set S \iassign h(\pcn)
+ \cup
+ \bigcup_{\p \in \allpatches}
+ \bigcup_{H \in h(\pn) \lor H \in h(\py)}
+ \{ \baseof{E} \; | \; E \in \pendsof{H}{\pcy} \}
+$$
+
+and $W \iassign w(h(\pcn))$
+
+\item While $\exists_{S \in \set S} S \ge W$,
+update $W \assign S$ and $\set S \assign \set S \, \backslash \{ S \}$
+
+(This will often remove $W$ from $\set S$. Afterwards, $\set S$
+is a collection of heads to be merged into $W$.)
+
+\item Choose an ordering of $\set S$, $S_i$ for $i=1 \ldots n$.
+
+\item For each $S_i$ in turn, choose a corresponding $M_i$
+such that $$
+ M_i \le S_i \land \left[
+ M_i \le W \lor \bigexists_{j<i} M_i \le S_j
+ \right]
+$$
+
+\item Set $\Gamma \iassign \depsreqof{W}$.
+
+If there are multiple candidates we prefer $M_i \in \pcn$
+if available.
+
+\item For each $i \ldots 1..n$, update our putative direct
+dependencies:
+$$
+\Gamma \assign \text{\bf set-merge}\left[\Gamma,
+ \left( \begin{cases}
+ M_i \in \pcn : & \depsreqof{M_i} \\
+ M_i \not\in \pcn : & \{ \}
+ \end{cases} \right),
+ \depsreqof{S_i}
+ \right]
+$$
+
+TODO define {\bf set-merge}
+
+\item Finalise our putative direct dependencies
+$
+\Gamma \assign g(\pc, \Gamma)
+$
+
+\item For each direct dependency $\pd \in \Gamma$,
+
+\begin{enumerate}
+\item Add an edge $\pc \hasdirdep \pd$ to the digraph (adding nodes
+as necessary).
+If this results in a cycle, abort entirely (as the function $g$ is
+inappropriate; a different $g$ could work).
+\end{enumerate}
+\item Run ${\text{\bf Rank-Recurse}}(\pd)$.
+
+\end{enumerate}
+
+\subsection{Results of the ranking phase}
+
+By the end of the ranking phase, we have recorded the following
+information:
+
+\begin{itemize}
+\item
+$ \allpatches, \hasdirdep $ and hence the completion of $\hasdirdep$
+into the partial order $\hasdep$.
+
+\item
+For each $\pc \in \allpatches$,
+the base branch starting point commit $W^{\pcn} = W$.
+
+\item
+For each $\pc$,
+the direct dependencies $\Gamma^{\pc} = \Gamma$.
+
+\item
+For each $\pc$,
+the ordered set of base branch sources $\set S^{\pcn} = \set S,
+S^{\pcn}_i = S_i$
+and corresponding merge bases $M^{\pcn}_i = M_i$.
+
+\end{itemize}
+
+\section{Traversal phase}
+
+
+
+
+