Error: Node instance "ul" instantiates undefined entity "xor2"

源程序代码如下：

LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
ENTITY exa_7_16 IS
PORT(ina,inb:IN STD_LOGIC;
ouc:OUT STD_LOGIC);
END exa_7_16;
ARCHITECTURE example OF exa_7_16 IS
COMPONENT xor2
GENERIC(ti:TIME);
PORT(a:IN STD_LOGIC;
b:IN STD_LOGIC;
c:OUT STD_LOGIC);
END COMPONENT;
BEGIN
ul:xor2 GENERIC MAP(5ns)
PORT MAP(ina,inb,ouc);
END example;