fact: Definition gtree_empty4 t := if t is GtreeNode t0 t1 t2 t3 then gtree_empty_and t0 (gtree_empty_and t1 (gtree_empty_and t2 (gtree_empty t3))) else false.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq) fourcolor(color chromogram)