Hint Resolve joinl_hom1.

Lemma joinl_top n (a: vect n) l1 l2 :
(forall i, In i (a :: l1) -> cbl _ (base n 1) i) ->
joinl _ (l1 ++ (a::l2)) = (-(1))^length l1 .* joinl _ (a:: (l1 ++ l2)%list).