if (
	    ($out_port == 1)
	    and
	    ($in_port  == 1)
	    )
	{			# single port	    
	    if ($out_causality eq $gain_causality)
	    if ($in_causality eq $gain_causality)
	    {
		$retval = "(($input)*($gain))";
	    }	    
	    elsif ($out_causality ne $gain_causality)
	    elsif ($in_causality ne $gain_causality)
	    {
		$retval = "(($input)/($gain))";
	    }
	}
    }
    
    if ($retval)