>>30

struct ListElement {
　struct ListElement *prev, *next;
　struct data *body;
};

struct ListElement head = {&head, &head, NULL};

void insert(struct ListElement *e)
{
　e->prev = head;
　e->next = head.next;
　head.next->prev = e;
　head.next = e;
}

void remove(struct ListElement *e)
{
　e->prev->next = e->next;
　e->next->prev = e->prev;
}

int empty(void)
{
　return head.next == &head;
}

void search(struct data *x)
{
　struct ListElement *l = head.next;

　while (l != &head)
　　if (l->body == x)
　　　return l;

　return NULL;
}