class Exp {
  Tokens tokens;
  public Exp parse(Tokens tokens) throws SyntaxError {
    this.tokens = tokens;
    return parseExp(null);
  }
  public Exp parseExp(Exp left) throws SyntaxError {
    if (tokens.atEnd()) 
      if (left == null) 
	throw new SyntaxError ("expression");
      else return left;
    else 
      if (tokens.nextIs("+")) {
	tokens.eat("+");
	Exp right = parseExp(null);
	return parseExp(new Plus(left, right));
      } else {
	  if (tokens.nextIs("*")) {
	    tokens.eat ("*");
	    Exp right = parseExp(null);
            return parseExp(new Times(left, right));
	  } else {
	      if (tokens.nextIs("(")) {
			tokens.eat("(");
			Exp subexp = parseExp(null);
			tokens.eat(")");
			return parseExp(subexp);
	      } else {
		  if (tokens.nextIs(")")) {
			return left;
		} 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() {}
}