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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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