fact: Fixpoint der_rec n := if n is n'.+1 then if n' is n''.+1 then n' * (der_rec n'' + der_rec n') else 0 else 1.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype) mathcomp(ssrnat seq path div) mathcomp(fintype tuple finfun bigop finset binomial)