bool withdrawal(struct account* acct, int amt) {
    if (acct->bal < amt) {
        return false;
    pthread_mutex_lock(&acct->mutex);
    if (acct->bal < amt) {
        pthread_mutex_unlock(&acct->mutex);
        return false;
    }
    acct->bal -= amt;
    pthread_mutex_unlock(&acct->mutex);
    return true;
}