Given a key, return the actual key stored in self that matches.
		Raise KeyError if the key isn't found.

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)