Set Bullet Behavior "Strict Subproofs".
Opaque upto.

Lemma make_shares_app : forall i l1 l2 shs, Zlength l1 + Zlength l2 <= Zlength shs ->
make_shares shs (l1 ++ l2) i =
make_shares shs l1 i ++ make_shares (sublist (Zlength l1) (Zlength shs) shs) l2 i.