fact: Definition submatrix T m n p q (f : 'I_p -> 'I_m) (g : 'I_q -> 'I_n) (M : 'M[T]_(m,n)) := \matrix_(i < p, j < q) M (f i) (g j).
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq path ssralg) mathcomp(fintype perm choice finfun matrix bigop zmodp poly mxpoly)