proposition: Lemma in_extend_grect p (r : grect) : subpred (predU1 p r) (extend_grect p r).
proof: Proof. case: r p => x0 x2 y0 y2 [x1 y1] [x y] /predU1P[->|] /=. by rewrite !leIx !ltxU !lexx !ltr_addl !orbT. by rewrite !leIx !ltxU -andbA => /and4P[-> -> -> ->]. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)