sclass ShuntingYardParser_v2 > ShuntingYardCore is Steppable {
  // input
  LS tok;
  int iTok = 1;
  
  // transitional
  int iLiteralStart;

  *() {}
  *(S s) { this(noTok(s)); }
  *(LS *tok) {}
  
  run { stepAll(this); }
  
  public bool step() {
    if (iTok >= l(tok)) {
      finishLiteral();
      handleEOT();
      false;
    }
    
    S t = tok.get(iTok);
    bool space = isSpacer(t);

    if (!space) {
      if (isOperatorOrBracket(t)) {
        finishLiteral();
        handleToken(t);
      } else
        if (iLiteralStart == 0)
          iLiteralStart = iTok;
    }

    iTok += 2;
    true;
  }
  
  @Override int precedence(S op) {
    if (eqOneOf(op, "*", "/")) ret 2;
    if (eqOneOf(op, "+", "-")) ret 1;
    ret 0;
  }
  
  bool isSpacer(S t) {
    ret emptyAfterTrim(t);
  }
  
  @Override bool isOperator(S t) {
    ret !isSpacer(t)
      && !startsWithLetterOrDigit(t) && !eq(t, ".");
  }
  
  void finishLiteral {
    if (iLiteralStart == 0) ret;
    S s = joinSubList(tok, iLiteralStart, iTok-1);
    if (!isSpacer(s))
      handleToken(s);
    iLiteralStart = 0;
  }
  
  // returns list that can be processed with CalcRPN
  LS get() {
    run();
    ret outputQueue;
  }
}