fact: Definition ctree_merge_pair tu tu' := let: CtreePair t0 t1 := tu in let: CtreePair t2 t3 := tu' in CtreePair (ctree_cons t1 t2 t3) (ctree_cons t0 t3 t2).
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq) fourcolor(color ctree dyck)