def changed():
    """ Mark the session as changed. A user of a session should
        call this method after he or she mutates a mutable object that
        is *a value of the session* (it should not be required after
        mutating the session itself).  For example, if the user has
        stored a dictionary in the session under the key ``foo``, and
        he or she does ``session['foo'] = {}``, ``changed()`` needn't
        be called.  However, if subsequently he or she does
        ``session['foo']['a'] = 1``, ``changed()`` must be called for
        the sessioning machinery to notice the mutation of the
        internal dictionary."""
