Can this be written as is_false(is_negative()) and reuse is_negative()?

Thanks @isuruf for reviewing. Yes, it should be possible.

This turns out to be problematic. Both is_false(is_negative(expr)) and is_false(is_nonnegative(expr)) can be true in the case where expr is non-real. I can go ahead with this change, but then I would need to check is_real first.

The option to this is to let is_negative etc throw for non-real numbers. My current idea for all is_ has been to allow all numeric input and throw for Sets, Logic etc.

What does sympy do?

Sympy does not throw for non-reals:

>>> import sympy
>>> sympy.I.is_positive
False

So either what we have in the PR now or first check is_real then is_negative results can be inverted to get is_nonnegative for free. What do you think @isuruf?