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 _init_signals(self):
        """Initializes the backends and gtk signals """
        self.connect("cursor-changed", self.on_select_row)
        _signals = BackendSignals()
        _signals.connect(_signals.BACKEND_ADDED, self.on_backend_added)
        _signals.connect(_signals.BACKEND_STATE_TOGGLED,
                         self.on_backend_state_changed)