proposition: Lemma gtree_sub_node_0 bs t0 t1 t2 t3 (et : colseq) : (size et).-1 = 0 -> gtree_sub (GtreeNode t0 t1 t2 t3) bs et = 0.
proof: Proof. case: et => [|e [|e' et]] // _; rewrite /gtree_sub /=. have Htf t: (if t is GtreeEmpty then false else false) = false by case: t. by rewrite !Htf; case: e => //; case: bs. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq) fourcolor(color chromogram)