function factorial2( n ) {
	return addon( n );
}
Evaluates the double factorial of `n`.

@private
@param {number} n - input value
@returns {(NonNegativeInteger|number)} double factorial

@example
var v = factorial2( 3 );
// returns 3

@example
var v = factorial2( 4 );
// returns 8

@example
var v = factorial2( 5 );
// returns 15

@example
var v = factorial2( 301 );
// returns Infinity