LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
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PACKAGE MY_PACKAGE IS
TYPE ROM IS ARRAY(0 TO 7) OF STD_LOGIC_VECTOR(3 DOWNTO 0);
END MY_PACKAGE;
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LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
USE IEEE.STD_LOGIC_ARITH.ALL;
USE WORK.MY_PACKAGE.ALL;
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ENTITY ROMS IS
PORT(ADDR:IN STD_LOGIC_VECTOR(3 DOWNTO 0);
  DATA:OUT STD_LOGIC_VECTOR(3 DOWNTO 0));
END ROMS;
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ARCHITECTURE ROMS OF ROMS IS
CONSTANT MY_ROM1:ROM:=("0000","0001","0010","0011","0100","0101","0110","0111");
CONSTANT MY_ROM2:ROM:=("1000","1001","1010","1011","1100","1101","1110","1111");
SIGNAL SEL: STD_LOGIC;
SIGNAL ADD: STD_LOGIC_VECTOR(2 DOWNTO 0);
SIGNAL AD : INTEGER;
BEGIN
SEL<=ADDR(3);
ADD<=ADDR(2 DOWNTO 0);
AD <=CONV_INTEGER(AD);

CASE SEL IS
  WHEN '0' => DATA<=MY_ROM1(AD);
  WHEN '1' => DATA<=MY_ROM2(AD);
  WHEN OTHERS => DATA<="ZZZZ";
END CASE;
END ROMS;

这是我练习的程序，就是模仿ROM，输入一个地址，输出对应处的数据。但是在case那里总是出错：
Error (10500): VHDL syntax error at ROMS.vhd(29) near text "CASE";  expecting "end", or "(", or an identifier ("case" is a reserved keyword), or a concurrent statement
Error (10500): VHDL syntax error at ROMS.vhd(31) near text "WHEN";  expecting "end", or "(", or an identifier ("when" is a reserved keyword), or a concurrent statement
类似这种错误。
我一开始用if语句，就一直出这种错，改了几次后，改成case，还是这样 expecting "end", or "(", or an identifier的错误。
求助！！谢谢！！