Definition is_base vs := free vs /\ forall e, cbl vs e.

Lemma addE_mprod ks1 ks2 vs : length ks1 = length ks2 -> map2 (fun k1 k2 => (k1 + k2)%f) ks1 ks2 *X* vs = ks1 *X* vs + ks2 *X* vs.