case OP_ADD: {
    Value b = pop_value(vm);
    Value a = pop_value(vm);
    if (a.type == VAL_INT && b.type == VAL_INT) {
        Value res = make_int(a.i + b.i);
        free_value(a);
        free_value(b);
        push_value(vm, res);
    } else if (a.type == VAL_STRING && b.type == VAL_STRING) {
        const char *sa = a.s ? a.s : "";
        const char *sb = b.s ? b.s : "";
        size_t la = strlen(sa);
        size_t lb = strlen(sb);
        char *buf = (char*)malloc(la + lb + 1);
        if (!buf) {
            fprintf(stderr, "Runtime error: out of memory during string concatenation\n");
            exit(1);
        }
        memcpy(buf, sa, la);
        memcpy(buf + la, sb, lb);
        buf[la + lb] = '\0';
        Value res;
        res.type = VAL_STRING;
        res.s = buf;
        free_value(a);
        free_value(b);
        push_value(vm, res);
    } else if (a.type == VAL_ARRAY && b.type == VAL_ARRAY) {
        Value res = array_concat(&a, &b);
        free_value(a);
        free_value(b);
        push_value(vm, res);
    } else {
        fprintf(stderr, "Runtime type error: ADD expects both ints, both strings, or both arrays, got %s and %s\n",
                value_type_name(a.type), value_type_name(b.type));
        exit(1);
    }
    break;
}