fact: Definition chordless (r : seq G) := let non_adj_r x := [predD1 [predD1 r & prev r x] & next r x] in r \subset [pred x | [disjoint adj x & non_adj_r x]].
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap jordan)