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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 pset_field(item_type, optional=False, initial=()):
 """ Create checked ``PSet`` field. :param item_type: The required type for the items in the set. :param optional: If true, ``None`` can be used as a value for this field. :param initial: Initial value to pass to factory if no value is given for the field. :return: A ``field`` containing a ``CheckedPSet`` of the given type. """

    return _sequence_field(CheckedPSet, item_type, optional,
                           initial)