code: def pvector_field(item_type, optional=False, initial=()):
      
      return _sequence_field(CheckedPVector, item_type, optional,
                             initial)
docs: Create checked ``PVector`` field.
  
      :param item_type: The required type for the items in the vector.
      :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 ``CheckedPVector`` of the given type.