<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_key(self, key='C'):
 """Add a key signature event to the track_data."""

        if isinstance(key, Key):
            key = key.name[0]
        self.track_data += self.key_signature_event(key)