proposition: Lemma fband_rev p : fband (rev p) =i fband p.
proof: Proof. by move=> x; apply: {x} eq_has_r => x; apply: mem_rev. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap jordan)