fact: Fixpoint vect_n_k n k := if k is k'.+1 then flatten (mkseq (fun i => map (cons i) (vect_n_k (n-i) k') ) n.+1) else if n is 0 then [:: [::]] else [::].
imports: mathcomp.ssreflect.ssreflect mathcomp(ssrbool ssrfun ssrnat eqtype seq) tools