+ *
+ * @cb is of form "void @cb(struct invbloom *ib, size_t bucket, bool
+ * before, data)". @before is true if the call is done before the
+ * singleton in @bucket is removed (ie. ib->counts[bucket] is -1 or 1,
+ * but is about to change). @before is false if the call is done
+ * after the operation, and the bucket is now a singleton.