Buggy exponent expression

[Xi Chan]

Insufficient checks are being performed for exponent operator. Also, some results are wrong.

• **(-(1), 0.5) yields NaN instead of 1j
• **(0, -(1)) crashes program
• **(0, 0) yields 1 (is it correct? - same in Python 3.3 and 2.7 interpreter) etc.