a past master的用法和样例：
- Henry is a past master at bridge.
- She's a past master at the art of getting what she wants.
- Trinket had always been a past master at this kind of skulduggery.
- The spy, a past master of machination and maneuver, was caught at last.
- After working in the factory for 35 years,this veteran worker is a past master in his line.