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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 add_item(self, item, field_name=None):
 """ Add the item to the specified section. Intended for use with items of settings.ARMSTRONG_SECTION_ITEM_MODEL. Behavior on other items is undefined. """

        field_name = self._choose_field_name(field_name)
        related_manager = getattr(item, field_name)
        related_manager.add(self)