function factorialln( x ) {
	return addon( x );
}
Evaluates the natural logarithm of the factorial of `x`.

@private
@param {number} x - input value
@returns {number} natural logarithm of factorial of `x`

@example
var v = factorialln( 3.0 );
// returns ~1.792

@example
var v = factorialln( 2.4 );
// returns ~1.092

@example
var v = factorialln( -1.0 );
// returns NaN

@example
var v = factorialln( -1.5 );
// returns ~1.266

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