- having to realloc and copy a giant bit array. The `prev' field is
- undefined for an element on the free list. */
+ having to realloc and copy a giant bit array.
+
+ The free list is implemented as a list of lists. There is one
+ outer list connected together by prev fields. Each element of that
+ outer is an inner list (that may consist only of the outer list
+ element) that are connected by the next fields. The prev pointer
+ is undefined for interior elements. This allows
+ bitmap_elt_clear_from to be implemented in unit time rather than
+ linear in the number of elements to be freed. */