> Are you sure we haven't such tests before?

In ```assert Integral(x*y*z, (x, 1, 2), (y, 1, 3)).is_number is False```, ```z``` is left without defining a limit for integration. Other counterparts are not in this forms.
E.g.:
```assert Integral(x*y, (x, 1, 2), (y, 1, z)).is_number is False``` gives rise to ```z```ed but not leave aside a variable at all.

In ```assert Integral(x*y*z, (x, 1, 2), (y, 1, 3), (z, 1, 3)).is_number is True```, the remedy to the above test (not the E.g.) is rendered. Further I could not see three variables integration which results in an integer (number).