presto-main/src/main/java/io/prestosql/operator/scalar/MapSubscriptOperator.java/MapSubscriptOperator/MapSubscriptOperator
class MapSubscriptOperator:    public MapSubscriptOperator()
    {
        super(SUBSCRIPT,
                ImmutableList.of(typeVariable("K"), typeVariable("V")),
                ImmutableList.of(),
                new TypeSignature("V"),
                ImmutableList.of(mapType(new TypeSignature("K"), new TypeSignature("V")), new TypeSignature("K")),
                true);
    }