+ every maximal Ce < C in Px
+ ie every Ce for which
+ C > Ce and !\exists C s.t. C > Ce
+
+existence of corresponding base B for every tip commit C
+ \forall C s.t. P(C) = Ptip
+ \exists B s.t. E(C,Pbase) = { B }
+ resulting function B(C)
+
+contents of tip commits
+ \forall C s.t. P(c) = Ptip
+ cont(C) = cont(B(C)) \union { Cp s.t. P(Cp) = Ptip and Cp < C }