proposition: Lemma D4can : cancel3 D4_edge D4_node D4_face.
proof: Proof. move=>x; apply/eqP; move: x; apply/forallP. by rewrite forall_all_ord. Qed.
imports: mathcomp(all_ssreflect) fourcolor(hypermap geometry) GraphTheory(preliminaries digraph sgraph) GraphTheory(hmap_ops embedding)