fact: Fixpoint ctree_union tl tr {struct tr} := match tl, tr with | CtreeNode tl1 tl2 tl3, CtreeNode tr1 tr2 tr3 => ctree_cons (ctree_union tl1 tr1) (ctree_union tl2 tr2) (ctree_union tl3 tr3) | CtreeEmpty, _ => tr | _, CtreeEmpty => tl | _, _ => ctree_simple_leaf end.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq) fourcolor(color)