nullify and empty: convert to seq_info.

Issues: (nullify 1..1) doesn't return nil as it
should, and (empty 1) fails.

* lib.c (seq_info): Since we would like nullify to use
seq_info, seq_info cannot call nullify. Transplant the probing
of the nullify method out of nullify and into here.
(nullify): Obtain a seq_info_t on the object. If it's not a
sequence and is not range, then just return the object.
Otherwise obtain a seq_iter_t sequence and peek whether it has
a next item.
(empty): Similarly, obtain an iterator and peek.
Definition of empty is relocated to be next to nullify.

* lib.h: Relocate declaration of empty, also.

1 files changed, 2 insertions, 2 deletions

diff --git a/lib.h b/lib.h
index 770fefc7..dff5f178 100644
--- a/lib.h
+++ b/lib.h
@@ -617,7 +617,8 @@ val push(val v, val *plist);
 val copy_list(val list);
 val make_like(val list, val thatobj);
 val tolist(val seq);
-val nullify(val seq);
+val nullify(val obj);
+val empty(val seq);
 val seqp(val obj);
 val nreverse(val in);
 val reverse(val in);
@@ -1166,7 +1167,6 @@ val isec(val list1, val list2, val testfun, val keyfun);
 val uni(val list1, val list2, val testfun, val keyfun);
 val copy(val seq);
 val length(val seq);
-val empty(val seq);
 val sub(val seq, val from, val to);
 val ref(val seq, val ind);
 val refset(val seq, val ind, val newval);