+\stdsection{ WIP tip satisfaction, reachable commits }
+
+Set of all reachable commits, O.
+
+A reachable commit is one we might refer to explicitly in any of these
+algorithms, and any ancestor of such a commit. We explicitly
+enumerate all of the input commits (U). And each commit we generate
+will be descended from zero or more of these.
+
+Naturally this set varies over time as we generate more commits. We
+write O_pyn for the set of reachable commits at the point where we
+have just generated T_pyn.
+
+We preserve/ensure
+ Tip_Pyn >= pendsof( O_pyn, Pyn )
+(Tip_pyn is computed during traversal for the patch p)
+
+We ensure this property by:
+ - we do not generate any commits for pyn other than
+ during traversal for pyn
+ - so at the start of traversal pendsof ( O, pyn) = pendsof (U, pyn)
+ - the traversal steps will takes those pends
+
+ - during traversal for the patch set pyn
+ pendsof
+
+ - arranging that during traversal for the patch set pyn
+
+, we firstly
+ compute a tip_pyn which is >= pendsof(O) as follows:
+ there is a set of relevant ends
+ pendsof(U,
+
+ we initially set T
+
+ by virtue of it being >= pendsof(U) \cup
+ all lower tip_qyn
+ -
+
+We do this by:
+ - firstly, Tip_pyn is only valid af
+
+
+Set of all reachable commits in a particular patch set.
+ This consists of
+ - input commits
+ -
+