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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 mark_flags_as_mutual_exclusive(flag_names, required=False, flag_values=FLAGS):
 """Ensures that only one flag among flag_names is set. Args: flag_names: [str], a list of the flag names to be checked. required: Boolean, if set, exactly one of the flags must be set. Otherwise, it is also valid for none of the flags to be set. flag_values: An optional FlagValues instance to validate against. """


  def validate_mutual_exclusion(flags_dict):
    flag_count = sum(1 for val in flags_dict.values() if val is not None)
    if flag_count == 1 or (not required and flag_count == 0):
      return True
    message = ('%s one of (%s) must be specified.' %
               ('Exactly' if required else 'At most', ', '.join(flag_names)))
    raise ValidationError(message)

  register_multi_flags_validator(
      flag_names, validate_mutual_exclusion, flag_values=flag_values)