fact: Definition vs2mx_sum_expr_subproof (S : addv_expr) : mxsum_spec (vs2mx (unwrap S)) (unwrap (addv_dim S)).
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype bigop finfun tuple) mathcomp(ssralg matrix mxalgebra zmodp)