function atan2d( y, x ) {
	return addon( y, x );
}
Computes the angle in the plane (in degrees) between the positive x-axis and the ray from `(0,0)` to the point `(x,y)`.

@private
@param {number} y - `y` coordinate
@param {number} x - `x` coordinate
@returns {number} angle (in degrees)

@example
var v = atan2d( 2.0, 2.0 ); // => atand(1.0)
// returns ~45.0

@example
v = atan2d( 6.0, 2.0 ); // => atand(3.0)
// returns ~71.565

@example
v = atan2d( 3.0, 0.0 );
// returns 90.0

@example
v = atan2d( 3.0, NaN );
// returns NaN