- * IMPORTANT: you MUST NOT call ANY other adns function with the same
- * adns_state, or with a query in the same adns_state, while you are
- * doing one of these iterations. After such a call the iterator
- * value has undefined meaning and must not be used.
+ * You can only have one iteration going at once. You may call _begin
+ * at any time; after that, an iteration will be in progress. You may
+ * only call _next when an iteration is in progress - anything else
+ * may coredump. The iteration remains in progress until _next
+ * returns 0, indicating that all the queries have been walked over,
+ * or ANY other adns function is called with the same adns_state (or a
+ * query in the same adns_state). There is no need to explicitly
+ * finish an iteration.
+ *
+ * context_r may be 0.