a troop of cavalry的用法和样例：
- They were taken in flank by a troop of cavalry.
- We were taken in flank by a troop of cavalry.
- There is a troop of cavalry guarding the borders on the ice-capped plateau.
- There is a troop of monkeys in the woods.
- He came out with a troop of followers.
- The old man was surrounded by a troop of children.