int expr(KeySet K) {

    int leftOp, rightOp;
    Symbol operator;
    leftOp = cmp(K);
    while (lex_symbol == PLUS || lex_symbol == MINUS) {
        operator = lex_symbol;
        next_symbol();
        rightOp = cmp(K);
        switch (operator) {

            case PLUS:
                leftOp = leftOp + rightOp;
                write_add();
                break;

            case MINUS:
                leftOp = leftOp - rightOp;
                write_sub();
                break;

            default:
                fprintf(stderr, "Neocakavany operator v expr()");
                exit(1);
        }

    }
    //printf("expr lefOp: %d\n", leftOp);
    //printf("expr rightOp: %d\n",rightOp);

    return leftOp;
}