Get the start key of the first range.

		None if RangeMap is empty or unbounded to the left.

def start(self):
		"""Get the start key of the first range.

		None if RangeMap is empty or unbounded to the left.
		"""
		if self._values[0] is NOT_SET:
			try:
				return self._keys[1]
			except IndexError:
				# This is empty or everything is mapped to a single value
				return None
		else:
			# This is unbounded to the left
			return self._keys[0]