code: Expr greaterLess() {
  		Expr expr = addSub();
  		Tok tok = peek();
  		if (tok.sym == Sym.LT || tok.sym == Sym.LE || tok.sym == Sym.GT || tok.sym == Sym.GE) {
  			move();
  			return new Compare(tok.sym, expr, addSub(), location);
  		}
  		return expr;
  	}
docs: compare expr ('<=' | '>=' | '>' | '<') expr
  不支持无限连： > >= < <=