proposition: Lemma extends_in_sub (r1 r2 : grect) : subpred r1 r2 -> forall p, extends_in r1 p -> extends_in r2 p.
proof: Proof. move=> sr12 p [xm xmP sxmr xm_p]; exists xm => {p xm_p}// p /sxmr. by rewrite !inE -(orb_idl (sr12 p)) orbAC => ->. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(path order ssralg ssrnum ssrint div intdiv) fourcolor(grid)