struct block {
   block *prev;
   block *next;
   // data goes here
   int data;
};

struct list {
   block *start;
   block *end;
   int length;
};

// list operations:
//   list_unlink: unlinks the last block in the list, and returns a pointer to it
block *list_unlink (list *l);
//   list_link:   links the block b onto the end of the list
void   list_link   (list *l, block *b);
//   list_init:   links an array of blocks onto the end of a list
void   list_init   (list *l, block *m, int size);
//   list_print:  prints a list
void   list_print  (list *l);