a flock mattress的用法和样例：
- A flock of sheep poured through the gate.
- In the forest, we came upon a flock of pheasants.
- We saw a flock of geese swimming in the lake.
- A flock of wild geese flew overhead.
- A flock of customers waited for the store to open.
- A flock of gulls circled overhead as the boast sailed into the port.