Args:
            graph (Fst Acceptor): The DFA
            cur_state (Fst State): The current State
            char (Char): The input character
        Returns:
            (Fst State): The destination state

def _delta(self, graph, cur_state, char):
        """
        Args:
            graph (Fst Acceptor): The DFA
            cur_state (Fst State): The current State
            char (Char): The input character
        Returns:
            (Fst State): The destination state
        """
        for arc in cur_state.arcs:
            if graph.isyms.find(arc.ilabel) == char:
                return graph[arc.nextstate]
        return None