Added the functions notmuch_tags_regress and notmuch_tags_is_first to
notmuch library. With them is possible to iterate backwards on tags.

* notmuch_tags_regress do the opposite than notmuch_tags_advance,
  getting the tags iterator one position backwards.

* notmuch_tags_is_first return TRUE if the iterator is in the first
  tag.
---
 lib/notmuch.h |    8 ++++++++
 lib/tags.c    |   19 +++++++++++++++++++
 2 files changed, 27 insertions(+), 0 deletions(-)

diff --git a/lib/notmuch.h b/lib/notmuch.h
index e28ce46..db051c8 100644
--- a/lib/notmuch.h
+++ b/lib/notmuch.h
@@ -917,6 +917,10 @@ notmuch_message_destroy (notmuch_message_t *message);
 notmuch_bool_t
 notmuch_tags_has_more (notmuch_tags_t *tags);
 
+/* Is the given notmuch_tags_t object on the first tags */
+notmuch_bool_t
+notmuch_tags_is_first (notmuch_tags_t *tags);
+
 /* Get the current tag from 'tags' as a string.
  *
  * Note: The returned string belongs to 'tags' and has a lifetime
@@ -936,6 +940,10 @@ notmuch_tags_get (notmuch_tags_t *tags);
 void
 notmuch_tags_advance (notmuch_tags_t *tags);
 
+/* Regress the 'tags' iterator to the previous result. */
+void
+notmuch_tags_regress (notmuch_tags_t *tags);
+
 /* Destroy a notmuch_tags_t object.
  *
  * It's not strictly necessary to call this function. All memory from
diff --git a/lib/tags.c b/lib/tags.c
index 85507e9..cf9e030 100644
--- a/lib/tags.c
+++ b/lib/tags.c
@@ -25,6 +25,7 @@
 struct _notmuch_tags {
     int sorted;
     GList *tags;
+    GList *previous_node;
     GList *iterator;
 };
 
@@ -55,6 +56,7 @@ _notmuch_tags_create (void *ctx)
 
     tags->sorted = 1;
     tags->tags = NULL;
+    tags->previous_node = NULL;
     tags->iterator = NULL;
 
     return tags;
@@ -94,6 +96,12 @@ notmuch_tags_has_more (notmuch_tags_t *tags)
     return tags->iterator != NULL;
 }
 
+notmuch_bool_t
+notmuch_tags_is_first (notmuch_tags_t *tags)
+{
+    return tags->previous_node == NULL;
+}
+
 const char *
 notmuch_tags_get (notmuch_tags_t *tags)
 {
@@ -109,10 +117,21 @@ notmuch_tags_advance (notmuch_tags_t *tags)
     if (tags->iterator == NULL)
 	return;
 
+    tags->previous_node = tags->iterator;
     tags->iterator = tags->iterator->next;
 }
 
 void
+notmuch_tags_regress (notmuch_tags_t *tags)
+{
+    if (tags->previous_node == NULL)
+	return;
+
+	tags->iterator = tags->previous_node;
+	tags->previous_node = tags->iterator->prev;
+}
+
+void
 notmuch_tags_destroy (notmuch_tags_t *tags)
 {
     talloc_free (tags);
-- 
1.6.5.4