proposition: Fact dnode_subproof (u : ddart) : node (val u) \in diskN.
proof: Proof. by rewrite -(fclosed1 diskN_node) (valP u). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap jordan color geometry patch)