proposition: Lemma skipP x (x_a : x \in a) : skip_spec x (skip x).
proof: Proof. rewrite skipE. set p := skip_exists _. case: ex_minnP => n N1 N2; constructor; auto. case/existsP : p => k Hk. apply: leq_trans (ltn_ord k). exact: N2. Qed.
imports: mathcomp(all_ssreflect) GraphTheory(edone preliminaries bij) fourcolor(hypermap geometry cfmap jordan color coloring combinatorial4ct) fourcolor(walkup)