a head of lettuce的用法和样例：
- We need a head of lettuce, a bunch of carrots, and a bag of onions.
- A can of tuna fish a loaf of white bread, and a head of lettuce.
- I'll take a pineapple, a head of cauliflower, two bunches of celery, and a head of lettuce.
- The President cannot rudely ignore a head of state.
- He that have a head of wax must not walk in the sun.