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