用VHDL写的程序，模拟狗，猫，老鼠过河。规则是：狗不能跟猫单独呆在一起，老鼠跟猫不能单独在一起。-Program written in VHDL, simulated dog, cat, mouse to cross the river. The rule is: the dog can not be with the cat alone with rats and cats can not be alone together.