atan2 returns the arc tangent of the result of dividing
the first argument by the second argument. Both arguments must be
real. This should be at least as accurate as, and probably faster
than, performing the division then using atan. The result is
expressed in radians, in the range
[- π, π]. If the arguments
are matricies, then they must have the same dimensions, and the operation
is performed element-by-element.