proposition: Lemma ctree_union_sym t1 t2 : ctree_union t1 t2 = ctree_union t2 t1.
proof: Proof. elim: t1 t2 => [t11 IH1 t12 IH2 t13 IH3|l1 IHl|] [t21 t22 t23|l2|] //=. by rewrite IH1 IH2 IH3. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq) fourcolor(color)