relations
	ancestry: of commits, a partial order
		C > D		C is a descendant of D
	inclusion: commit includes change made in another
		a partial order, consistent with ancestry but more partial
		C has D <=> D isin C
	C has D => C > D

define convenience function
	cont(C) := { Cx | Cx isin C }

patches
	patch P is two sets Px of commits, Ptip and Pbase
	each subset has a single ur-ancestor C0_Ptip resp C0_Pbase
	all these subsets Ptip,Qbase,Rtip,Sbase mutually exclusive
	P(C) is either Ptip, Pbase, or nil (recorded in commit)

commits fall into three categories wrt P
 inpatch(P,C) = included
	P included in C
		\forall D \elem Ptip,  C has D <=> C > D
 inpatch(P,C) = absent
	P absent from C
		\forall D \elem Ptip, !(C has D)
 inpatch(P,C) = removed
	P removed from C
		cont(C) \intersection Ptip = { }
		but maybe \exists Cx \elem Ptip s.t. Cx < C

the ends E(C,Px) of Px = Pbase or Ptip are
	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 }

non-circularity
	\forall B s.t. P(c) = Pbase
		cont(B) \notintersect Ptip

normal commits
 make C from D
 result
	C < { D }
	P(C) = P(D)
	Cx isin C  <=>  Cx = D  v  Cx isin D
	inpatch(P,C) = inpatch(P,D)

anticommit for removing a patch P from D to make C
 make C from D, A-
 where
	P(C) = Qbase
	P(A-) = Ptip
	inpatch(P,C) = included
	E(D,Ptip) = { A- }
 then let
	A+ = B(A-)
 hence
	A+ < A- < D
	E(D,Pbase) = { A+ }
 result
	C >1 { D }
	P(C) = P(D)
	inpatch(P,C) = removed

single patch dependency application
 make C from D, A+
 where
	P(C) = Qbase
	P(A+) = Ptip
	inpatch(P,C) != included
 then let
	A- = B(A+)
 hence
	A- < A+
 result
	C >1 { D, A+ } => C > D, C > A+ > A-
	P(C) = P(D)
	inpatch(P,C) = included

simple merge
 make C from L, R, M
 where
	M < L < C
	M < R < C
 result
	C >1 { L, R }
	P(C) = P(L)
	if Cx isin L = Cx isin R,
		Cx isin C = Cx isin L
	otherwise
		Cx isin C = ! Cx isin M
	
  cases of inpatch(P,L) and ,R
		
	  or	Cx isin