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

i've got the same error for the operator "-". Using signed type and ieee.numeric_std. I would like to ask if my usage of "for" is correct.

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.STD_LOGIC_signed.ALL;

use IEEE.STD_LOGIC_TEXTIO.ALL;

use ieee.numeric_std.all;

entity stage_1 is

Port (

clk : in bit;

imagem: in SIGNED (19999 DOWNTO 0);

imagemanterior: in SIGNED (19999 DOWNTO 0);

Ix: out SIGNED (24999 DOWNTO 0);

Iy: out SIGNED (24999 DOWNTO 0);

It: out SIGNED (24999 DOWNTO 0)

);

end stage_1;

architecture Behavioral of stage_1 is

begin

process (clk)

begin

If (clk 'event and clk = '1') then

for I in 0 to 2448 loop

Ix(I*10+9 DOWNTO I*10) <= 1/4 * (imagemanterior((I+1)*8+7 DOWNTO (I+1)*8) - imagemanterior(I*8+7 DOWNTO I*8) + imagemanterior ((I+1)*8+407 DOWNTO (I+1)*8+400) - imagemanterior(I*8+407 DOWNTO I*8+400) + imagem((I+1)*8+7 DOWNTO(I+1)*8) - imagem(I*8+7 DOWNTO I*8)+ imagem((I+1)*8+407 DOWNTO(I+1)*8) - imagem((I)*8+407 DOWNTO I*8 +400));

Iy(I*10+9 DOWNTO I*10) <= 1/4*(imagemanterior((I*8)+407 DOWNTO (I)*8+400) - imagemanterior(I*8+7 DOWNTO I*8) + imagemanterior((I+1)*8+407 DOWNTO (I+1)*8) - imagemanterior((I+1)*8+7 DOWNTO(I+1)*8) + imagem(I*8+407 DOWNTO I*8+400) - imagem(I*8+7 DOWNTO I*68) + imagem((I+1)*8+407 DOWNTO (I+1)*8+400) -imagem((I+1)*8+7 DOWNTO(I+1)*8));

It(I*10+9 DOWNTO I*10) <= 1/4 * (imagem(I*8+7 DOWNTO I*8)- imagemanterior((I*8+7)-I*8) + imagem((I*8)+407 DOWNTO I*8+400)- imagemanterior(I*8+407 DOWNTO I*8+400) +imagem((I+1)*8+7 DOWNTO(I+1)*8)- imagemanterior((I+1)*8+7 DOWNTO(I+1)*8) + imagem((I+1)*8+407 DOWNTO (I+1)*8+400)- imagemanterior((I+1)*8+407 DOWNTO (I+1)*8+400));

end loop;

end if ;

end process;

end Behavioral;