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, 5 insertions, 0 deletions

diff --git a/lib.c b/lib.c
index 58737f69..66142ec1 100644
--- a/lib.c
+++ b/lib.c
@@ -674,6 +674,11 @@ val nthcdr(val pos, val list)
   return list;
 }
 
+val nth(val pos, val list)
+{
+  return car(nthcdr(pos, list));
+}
+
 val nthlast(val pos, val list)
 {
   val iter = list;