def add(self, elem, replace=False):
        """Add a requirement.

        :param elem: a string or :class:`Requirement` instance.
        :param replace: if `True`, packages in the set with the same name will
            be removed first.
        """
        if isinstance(elem, text_type):
            elem = Requirement.coerce(elem)
        if replace and elem in self:
            self.remove(elem)
        super(Requirements, self).add(elem)
def _setup_drag(self, item):
    self._drag_and_drop_context.setup_drag(
      # Using the entire item allows dragging only by the label rather than the
      # widget itself. This avoids problems with widgets such as spin buttons
      # that do not behave correctly when reordering and also avoids accidental
      # clicking and modifying the widget by the user.
      item.widget,
      self._get_drag_data,
      self._on_drag_data_received,
      [item],
      [item],
      self)