def new(self, sort, properties=None):
        """
        Create a new variable for the given *sort*.
        """
        if sort is None:
            sort = UNKNOWNSORT
        # find next available vid
        vid, index = self.vid, self.index
        while vid in index:
            vid += 1
        varstring = '{}{}'.format(sort, vid)
        index[vid] = varstring
        if properties is None:
            properties = []
        self.store[varstring] = properties
        self.vid = vid + 1
        return (varstring, properties)