code: Expr addSub() {
  		Expr expr = mulDivMod();
  		for (Tok tok=peek(); tok.sym==Sym.ADD || tok.sym==Sym.SUB; tok=peek()) {
  			move();
  			expr = new Arith(tok.sym, expr, mulDivMod(), location);
  		}
  		return expr;
  	}
docs: addSub : expr ('+'|'-') expr