fact: Definition mx2 a b c d := block_mx (a%:M : 'M[R]_1) b%:M c%:M d%:M.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq path) mathcomp(ssralg fintype matrix mxalgebra bigop zmodp perm choice) dvdring mxstructure minor stronglydiscrete coherent edr