entity sum2 is
  generic ( k1, k2 : real := 1.0 );  -- optional gain multipliers
  port ( quantity in1, in2 : in real;
         quantity output : out real );
end entity sum2;

----------------------------------------------------------------

architecture simple of sum2 is
begin

  output == k1 * in1 + k2 * in2;  -- sum of inputs (with optional gain)

end architecture simple;