+ * The "delta" parameter to indexbuild_new is filled in with the
+ * maximum size by which the index can ever grow in a single
+ * indexbuild_add call. This can be used, together with
+ * indexbuild_realsize, to detect when the index is about to get
+ * too big for its file and the file needs resizing.
+ *
+ * indexbuild_add adds a trie_file structure to the index.
+ * indexbuild_tag, called after that, causes the current state of
+ * the index to be preserved so that it can be queried later. It's
+ * idempotent, and will safely do nothing if called before
+ * indexbuild_add.
+ *