function avercos( x ) {
	return addon( x );
}
Computes the inverse versed cosine of a double-precision floating-point number.

@private
@param {number} x - input value
@returns {number} inverse versed cosine

@example
var v = avercos( 0.0 );
// returns 0.0

@example
var v = avercos( -3.141592653589793/2.0 );
// returns ~2.1783

@example
var v = avercos( -3.141592653589793/6.0 );
// returns ~1.0742

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