new function: nth

Just the ANSI CL nth for lists.

* eval.c (eval_init): Register nth intrinsic.

* lib.c (nth): New function.

* lib.h (nth): Declared.

* share/txr/stdlib/place.tl (nth): New place macro,
trivially takes care of making nth an accessor.
Place macros are terrific!

* txr.1: Documented.

1 files changed, 1 insertions, 0 deletions

diff --git a/lib.h b/lib.h
index a9024443..9b8c997b 100644
--- a/lib.h
+++ b/lib.h
@@ -540,6 +540,7 @@ loc lastcons(val list);
 val last(val list, val n);
 val nthlast(val pos, val list);
 val nthcdr(val pos, val list);
+val nth(val pos, val list);
 val butlastn(val n, val list);
 loc ltail(loc cons);
 val pop(val *plist);