+All of these sets will be disjoint by construction
+(see Invariants, below). Hence:
+
+\item[ $\foreign$ ]
+The set of all commits which are not part of a Topbloke branch. We
+call these foreign commits.
+
+\item[ $\set A$, $\set P$, $\ldots$ ]
+Arbitrary sets of commits. Maybe $\set P = \p$ i.e.\ some $\py$ or $\pn$, but
+maybe not.