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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 validate(cls, mapper_spec):
 """Validates the mapper's specification and all necessary parameters. Args: mapper_spec: The MapperSpec to be used with this InputReader. Raises: BadReaderParamsError: If the user fails to specify both a starting time and an ending time, or if the starting time is later than the ending time. """

    if mapper_spec.input_reader_class() != cls:
      raise errors.BadReaderParamsError("Input reader class mismatch")

    params = _get_params(mapper_spec, allowed_keys=cls._PARAMS)
    if (cls.VERSION_IDS_PARAM not in params and
        cls.MODULE_VERSIONS_PARAM not in params):
      raise errors.BadReaderParamsError("Must specify a list of version ids or "
                                        "module/version ids for mapper input")
    if (cls.VERSION_IDS_PARAM in params and
        cls.MODULE_VERSIONS_PARAM in params):
      raise errors.BadReaderParamsError("Can not supply both version ids or "
                                        "module/version ids. Use only one.")
    if (cls.START_TIME_PARAM not in params or
        params[cls.START_TIME_PARAM] is None):
      raise errors.BadReaderParamsError("Must specify a starting time for "
                                        "mapper input")
    if cls.END_TIME_PARAM not in params or params[cls.END_TIME_PARAM] is None:
      params[cls.END_TIME_PARAM] = time.time()

    if params[cls.START_TIME_PARAM] >= params[cls.END_TIME_PARAM]:
      raise errors.BadReaderParamsError("The starting time cannot be later "
                                        "than or the same as the ending time.")

    if cls._PROTOTYPE_REQUEST_PARAM in params:
      try:
        params[cls._PROTOTYPE_REQUEST_PARAM] = log_service_pb.LogReadRequest(
            params[cls._PROTOTYPE_REQUEST_PARAM])
      except (TypeError, ProtocolBuffer.ProtocolBufferDecodeError):
        raise errors.BadReaderParamsError("The prototype request must be "
                                          "parseable as a LogReadRequest.")

    # Pass the parameters to logservice.fetch() to verify any underlying
    # constraints on types or values.  This only constructs an iterator, it
    # doesn't trigger any requests for actual log records.
    try:
      logservice.fetch(**params)
    except logservice.InvalidArgumentError, e:
      raise errors.BadReaderParamsError("One or more parameters are not valid "
                                        "inputs to logservice.fetch(): %s" % e)