code: def add_arc(self, src, dst, char):
          
          if src not in self.automaton.states():
              self.add_state()
          arc = fst.Arc(self.isyms[char], self.osyms[char],  fst.Weight.One(self.automaton.weight_type()), dst)
          self.automaton.add_arc(src, arc)
docs: Adds a new Arc
          Args:
              src (int): The source state identifier
              dst (int): The destination state identifier
              char (str): The character for the transition
          Returns:
              None