def add(self, key, value):
        """Add an entry to a list preference

        Add `value` to the list of entries for the `key` preference.

        """
        if not key in self.prefs:
            self.prefs[key] = []
        self.prefs[key].append(value)
Add an entry to a list preference

        Add `value` to the list of entries for the `key` preference.