fact: Variable find_pivot : forall m n, 'M[E]_(1 + m,1 + n) -> option ('I_(1 + m) * 'I_(1 + n)).
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq path) mathcomp(ssralg ssrint ssrnum fintype choice) mathcomp(matrix mxalgebra bigop zmodp perm) dvdring mxstructure stronglydiscrete coherent edr