fact: Definition pre_hom_ring x p := [/\ path rlink x p, {subset x :: p <= ac} & scycle rlink (map h (x :: p))].
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap geometry color chromogram coloring patch) fourcolor(snip revsnip kempe birkhoff contract)