fact: Fixpoint zeroExtendAux extra {n} (p: BITS n) : BITS (extra+n) := if extra is e.+1 then joinmsb0 (zeroExtendAux e p) else p.
imports: Coq(ZArith.ZArith Strings.String) mathcomp.ssreflect.ssreflect mathcomp(ssrfun ssrbool eqtype ssrnat seq fintype tuple zmodp)