fact: Fixpoint fromNegZ {n} (z: Z): BITS n := if n is _.+1 then joinlsb (fromNegZ (Z.div2 z), Zeven_bool z) else nilB.
imports: Coq(ZArith.ZArith Strings.String) mathcomp.ssreflect.ssreflect mathcomp(ssrfun ssrbool eqtype ssrnat seq fintype tuple zmodp)