void borrowBook(book_list* b1) {
    char tab[80];
    book* tmp;
    printf("Insert a title to borrow:\n");
    gets(tab);
    tmp= b1->first;
    do {
        if(strcmp(tab,tmp->title)==0)
            tmp->copies--;
        tmp=tmp->next;
    }while(tmp!=b1->first);
}