fact: Definition grec1down n := if (n.+1 - size init) is n'.+1 then gloop1 n'.+1 (size init) a0 (rev init) else rev (take n.+1 init).
imports: Coq(ZArith Rfunctions NaryFunctions Lia) mathcomp.ssreflect(ssreflect ssrbool ssrfun eqtype ssrnat seq fintype bigop tuple) MathComp