The standard two argument arctangent function. Since it interprets its
two arguments in comparison with one another, the input may have any
dimension.
>>> atan2 _0 _1
0.0
>>> atan2 _1 _0
1.5707963267948966
>>> atan2 _0 (negate _1)
3.141592653589793
>>> atan2 (negate _1) _0
-1.5707963267948966