proposition: Fact snipr_subproof : cancel3 redge rnode rface.
proof: Proof. move=> u; apply: val_inj; rewrite /= /snipr_node /snipr_face edgeK. have [ru | r'u] := boolP (val u \in r); first by rewrite mem_next ru prev_next. rewrite edgeK [_ \in r](contraNF _ (valP u)) // => rfeu. by rewrite inE /= r'u -[sval u]edgeK -(fclosed1 diskN_node) diskN_E inE /= rfeu. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap jordan color geometry patch)