proposition: Fact rface_subproof (u : rdart) : snipr_face (sval u) \notin diskE.
proof: Proof. rewrite /snipr_face. case: ifPn => [rnfu | r'nfu]; first by rewrite inE /= mem_next rnfu. apply: contra (valP u) => /andP[_ /= dNfu]; rewrite -[val u]faceK. by rewrite -(fclosed1 diskE_edge) !inE r'nfu -(fclosed1 diskN_node). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap jordan color geometry patch)