#[global]
Instance linear_union_ok s1 s2 `(Ok s1, Ok s2) :
Ok (linear_union s1 s2).
#[global]
Instance fold_add_ok s1 s2 `(Ok s1, Ok s2) :
Ok (fold add s1 s2).
#[global]
Instance union_ok s1 s2 `(Ok s1, Ok s2) : Ok (union s1 s2).

Lemma union_list_spec x l1 l2 acc :
InA X.eq x (union_list l1 l2 acc) <->
InA X.eq x l1 \/ InA X.eq x l2 \/ InA X.eq x acc.