fact: Definition diag_block_mx s F := if s is x :: l return 'M_((size_sum s).+1) then diag_block_mx_rec x l F else 0.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq path) mathcomp(ssralg fintype perm poly mxpoly finfun tuple) mathcomp(matrix bigop zmodp polydiv) ssrcomplements dvdring