let rec check_equal positive e1 e2 =
let one,zero =
if positive then Ival.singleton_one, Ival.singleton_zero else
Ival.singleton_zero, Ival.singleton_one
in
inject_ival
(if (equal e1 e2) && (cardinal_zero_or_one e1)
then one
else
if intersects e1 e2
then Ival.zero_or_one
else zero)