code: def start(self):
  		
  		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]
docs: Get the start key of the first range.
  
  		None if RangeMap is empty or unbounded to the left.