fact: Definition mesym (k : nat) : {mpoly R[n]} := \sum_(h : {set 'I_n} | #|h| == k) \prod_(i in h) 'X_i.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(choice fintype tuple finfun bigop finset binomial) mathcomp(order fingroup perm ssralg zmodp poly ssrint) mathcomp(matrix vector) mathcomp(bigenough) ssrcomplements freeg