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