$input,
	    $in_causality,
	    $in_port) = @args;

	if (($out_port == 2) and
	    ($in_port  == 1))
	{			# uni-causal
	    $retval = "((input)*(gain))";
	    $retval = "(($input)*($gain))";
	}

	elsif (($out_port == 1) and
	       ($in_port  == 2))
	{			# bi-causal
	    $retval = "((input)/(gain))";
	    $retval = "(($input)/($gain))";
	}
    }
    
    if ($retval)
    {
	return $retval;
    }