Membership test on the keys of a map.
Exp₁ in Exp₂
true if the value of Exp₁ occurs as key in the map value of Exp₂ and
The type of Exp₁ should be compatible with the key type TK of Exp₂.
rascal>"pear" in ("apple": 1, "pear": 2);
rascal>"pineapple" in ("apple": 1, "pear": 2);