function ahavercos( x ) {
	return 2.0 * acos( sqrt( x ) );
}
Computes the inverse half-value versed cosine.

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

@example
var v = ahavercos( 0.0 );
// returns ~3.1416

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

@example
var v = ahavercos( 0.5 );
// returns ~1.5708

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