# atan2 function

A variant of atan.

Converts both arguments to doubles.

Returns the angle in radians between the positive x-axis
and the vector (`b`

,`a`

).
The result is in the range -PI..PI.

If `b`

is positive, this is the same as `atan(a/b)`

.

The result is negative when `a`

is negative (including when `a`

is the
double -0.0).

If `a`

is equal to zero, the vector (`b`

,`a`

) is considered parallel to
the x-axis, even if `b`

is also equal to zero. The sign of `b`

determines
the direction of the vector along the x-axis.

Returns NaN if either argument is NaN.

## Implementation

`external double atan2(num a, num b);`