Two arbFs with same arguments always produce identical functions

[mrbackend]

Consider:

@RunWith(PropertyTestRunner.class)
public final class ArbFProperties {

    public Property arbFProperty() {
        return Property.property(
                Arbitrary.arbF(Cogen.cogenInteger, Arbitrary.arbInteger),
                Arbitrary.arbF(Cogen.cogenInteger, Arbitrary.arbInteger),
                Arbitrary.arbInteger,
                (f1, f2, arg) -> {
                    System.out.format("arg = %d, f1(arg) = %d, f2(arg) = %d%n", arg, f1.f(arg), f2.f(arg));
                    return Property.prop(true);
                });
    }

}

The functions always produce the same result. This seems to be intentional, the way arbF is programmed, but it is surprising to me. To create two independent functions, I have to use different cogenerators for the arguments (different arbitraries for the results are harder to make independent).

Is this a bug?

[mrbackend]

A workaround that seems to work ok is:

...
Arbitrary.arbF(Cogen.cogenInteger, Arbitrary.arbInteger),
Arbitrary.arbF(Cogen.cogenInteger.<Integer>contramap(i -> i + 1), Arbitrary.arbInteger),
...