class Exp {
  Tokens tokens;
  public Exp parse(Tokens tokens) throws SyntaxError {
    this.tokens = tokens;
    return parseExp(null);
  }
  private Exp parseExp(Exp exp) throws SyntaxError {
    if (tokens.atEnd()) {
      if (exp == null) 
	throw new SyntaxError ("expression");
      else return exp;
    }
    else if (tokens.nextIs("+")) {
      tokens.eat("+");
      Exp right = parseExp(null);
      return parseExp(new Plus(exp, right));
    }
    else if (tokens.nextIs("*")) {
      tokens.eat ("*");
      Exp right = parseExp(null);
      return parseExp(new Times(exp, right));
    }
    else if (tokens.nextIs("(")) {
      tokens.eat("(");
      Exp subexp = parseExp(null);
      tokens.eat(")");
      return parseExp(subexp);
    } 
    else if (tokens.nextIs(")")) {
      return exp;
    } 
    else {
      try {
        String s = tokens.next();
	int i = Integer.parseInt(s);
	return parseExp(new Constant(i));
      } 
      catch (NumberFormatException e) {
	throw new SyntaxError ("integer");
      }
    }
  }
  public void compile() {}
}