function acos( x ) {
	return addon( x );
}
Computes the arccosine of a double-precision floating-point number.

@private
@param {number} x - input value
@returns {number} arccosine (in radians)

@example
var v = acos( 1.0 );
// returns 0.0

@example
var v = acos( 0.707 ); // ~pi/4
// returns ~0.7855

@example
var v = acos( NaN );
// returns NaN