proposition: Lemma zpvalid_stepL zp : zpvalid zp -> zpvalid (zstepL zp).
proof: Proof. by case: zp => -[] // pl pr; do [apply: zpvalid_shiftl | apply: zpvalid_shiftr]. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap geometry part quiz quiztree)