<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def set(self, key, items):
 """Set key to a copy of items"""

        if not isinstance(items, list):
            raise ValueError("items must be a list")
        with self._lock:
            self._dict[key] = items.copy()