fact: Fixpoint nder_rec n := if n is n1.+1 then if n1 is n2.+1 then n1 *(nder_rec n1 + nder_rec n2) else 0 else 1.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype) mathcomp(ssrnat seq path div) mathcomp(fintype tuple finfun bigop finset binomial)