Definition is_decomposable n M := exists l, decomposable n l M.

(* Specifications: *)
Lemma one_factor0 n k : one_factor n k 0 = 0.
Lemma one_factor_hom n k1 k2 (x: vect n) : k2 < k1 -> hom n k1 x -> hom n (k1 - k2) (one_factor n k2 x).
Lemma one_factor_zero n k1 k2 (x: vect n) : k2 < k1 -> hom n k1 x -> one_factor n k2 x = 0 -> x = 0.