说明：  把时序逻辑电路设计和组合逻辑电路设计相结合，设计一个有实际应用的数字逻辑电路余3码转换成2421 BCD 码
(The sequential logic circuit design and the design of combinational logic circuit by combining the design of a practical application of digital logic circuits into three yards more than 2421 BCD code)