Set Bullet Behavior "Strict Subproofs".
Opaque upto.

Lemma make_shares_In : forall i lasts lgood x shs (Hx : 0 <= x < Zlength lasts)
(Hi : Znth x lasts 0 <> i) (Hgood : Znth x lgood false = true)
(Hlen : Zlength shs >= Zlength lasts),
In (Znth x shs Tsh) (make_shares shs (combine lgood lasts) i).