docs: def _LastEntryTimestamp(dct, upper_bound_timestamp):
      """Searches for greatest timestamp lower than the specified one.
  
      Args:
        dct: A dictionary from timestamps to some items.
        upper_bound_timestamp: An upper bound for timestamp to be returned.
  
      Returns:
        Greatest timestamp that is lower than the specified one. If no such value
        exists, `None` is returned.
      """
      if upper_bound_timestamp is None:
        upper_bound = lambda _: True
      else:
        upper_bound = lambda key: key <= upper_bound_timestamp
  
      try:
        return max(filter(upper_bound, iterkeys(dct)))
      except ValueError:  # Thrown if `max` input (result of filtering) is empty.
        return None