def clear_app_refs(self, gui=None):
        """Clear IPython's internal reference to an application instance.

        Whenever we create an app for a user on qt4 or wx, we hold a
        reference to the app.  This is needed because in some cases bad things
        can happen if a user doesn't hold a reference themselves.  This
        method is provided to clear the references we are holding.

        Parameters
        ----------
        gui : None or str
            If None, clear all app references.  If ('wx', 'qt4') clear
            the app for that toolkit.  References are not held for gtk or tk
            as those toolkits don't have the notion of an app.
        """
        if gui is None:
            self._apps = {}
        elif gui in self._apps:
            del self._apps[gui]
Clear IPython's internal reference to an application instance.

        Whenever we create an app for a user on qt4 or wx, we hold a
        reference to the app.  This is needed because in some cases bad things
        can happen if a user doesn't hold a reference themselves.  This
        method is provided to clear the references we are holding.

        Parameters
        ----------
        gui : None or str
            If None, clear all app references.  If ('wx', 'qt4') clear
            the app for that toolkit.  References are not held for gtk or tk
            as those toolkits don't have the notion of an app.
        
debugger/src/pydev_ipython/inputhook.py