a four-leaf clover的用法和样例：
- A four leaf clover is believed to bring good luck.
- A best friend is like a four leaf clover, hard to find and lucky to have.
- Best friend is like a four leaf clover, hard to find and lucky to have.
- Jenny: Come on! It's no different than a foreigner keeping a four leaf clover for good luck!
- The mystique of the four leaf clover continues today, since finding a real four leaf clover is still a rare occurrence and omen of good luck.