function atanh( x ) {
	if ( x === 0.0 ) {
		return x;
	}
	if (
		isnan( x ) ||
		isinfinite( x )
	) {
		return NaN;
	}
	return 0.5 * ln( (1.0+x) / (1.0-x) );
}
Computes the hyperbolic arctangent of a number.

@param {number} x - input value
@returns {number} hyperbolic arctangent (in radians)

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

@example
var v = atanh( 0.9 );
// returns ~1.472

@example
var v = atanh( 1.0 );
// returns Infinity

@example
v = atanh( -1.0 );
// returns -Infinity

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