std_ulogic_vector

Wie muss ich das folgende umwandeln um zwei std_ulogic_vectoren 
variabler länge zu addieren?

function "+" (
  constant l, r : std_ulogic_vector )
  return std_ulogic_vector is
  variable result : std_ulogic_vector(l'left downto l'right);

  begin

result := 
std_ulogic_vector(conv_std_logic_vector(unsigned(l)+unsigned(r), 
get_wid(l)));

return result;

end "+";