Error 10327: Can't determine definition of operator ""+""

I wite a code for multiplier and I have a problem with this erro " can't determine definition of operator ""+"". I have used almost library as you see and tried many ways that I know but it's still bad. Help me plz. can you help me re-write this code or show me a good way to solve. Thanks alot. I use Quatus II , Web edition

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.STD_LOGIC_ARITH.ALL;

use ieee.numeric_std.all;

use ieee.std_logic_signed.all;

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

entity mul is

port(a: in bit_vector(3 downto 0);

b: in bit_vector(3 downto 0);

kq: out bit_vector( 7 downto 0));

end mul;

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

architecture mul of mul is

type arr is array( 0 to 3) of bit_vector(7 downto 0);

begin

process(a,b)

variable x: arr;

variable t: bit_vector(7 downto 0);

begin

for j in 0 to 3 loop

if b(j)='1' then

x(j):=("0000"& a);

x(j):=(x(j) sll j);

else

x(j):=(others =>'0');

end if;

t:=(t+x(j));

end loop;

kq<=t;

end process;

end mul;