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

@private
@param {number} x - input value
@returns {number} hyperbolic arctangent

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

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