code: public MutableArc addArc(String startStateSymbol, String inSymbol, String outSymbol, String endStateSymbol, double weight) {
      Preconditions.checkNotNull(stateSymbols, "cant use this without state symbols; call useStateSymbols()");
      return addArc(
          getOrNewState(startStateSymbol),
          inSymbol,
          outSymbol,
          getOrNewState(endStateSymbol),
          weight
      );
    }
docs: Adds a new arc in the FST between startStateSymbol and endStateSymbol with inSymbol and outSymbol
  and edge weight; if the state symbols or in/out symbols dont exist then they will be added
  @param startStateSymbol
  @param inSymbol
  @param outSymbol
  @param endStateSymbol
  @param weight
  @return