docs: def matching_key_for(self, key):
  		"""
  		Given a key, return the actual key stored in self that matches.
  		Raise KeyError if the key isn't found.
  		"""
  		try:
  			return next(e_key for e_key in self.keys() if e_key == key)
  		except StopIteration:
  			raise KeyError(key)