bug#21782: 25.0.50; New functions nfront/front

[Tino Calancha <f92capac@gmail.com>]

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In GNU Emacs 25.0.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 2.24.23)
  of 2015-10-29
Repository revision: 07830c3d5c801d7d55622215b46ba692c6afa1d2


I found very useful to define one function returning:
(nreverse (last (nreverse list) N))
(for a given list and integer N)

Maybe its already defined somewhere, but i cannot find it.

Examples of the new function:
(let ((ltest '( 1 2 3 4 5 6)))
   (nfront ltest 3))
(1 2 3)

(let ((ltest '( 1 2 3 4 5 6)))
   (nfront ltest 1))
(1)

(let ((ltest '( 1 2 3 4 5 6)))
   (nfront ltest 20))
(1 2 3 4 5 6)

(let ((ltest '( 1 2 3 4 5 6)))
   (nfront ltest))
(1 2 3 4 5 6)

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diff --git a/lisp/subr.el b/lisp/subr.el
index ea926ae..976e4fd 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -405,6 +405,19 @@ nbutlast
 	   (if (> n 0) (setcdr (nthcdr (- (1- m) n) list) nil))
 	   list))))
 
+(defun front(list &optional n)
+  "Return a copy of LIST with just the first N elements.
+If N is omitted or nil the full list is copied."
+(let ((m (length list)))
+  (if (and n (<= n 0)) list
+    (nfront (copy-sequence list) n))))
+
+(defun nfront(list &optional n)
+  "Modified LIST to remove all elements but the first N.
+If N is omitted or nil the full list is copied."
+  (if (or (null n) (and n (<= n 0))) list
+    (nreverse (last (nreverse list) n))))
+
 (defun zerop (number)
   "Return t if NUMBER is zero."
   ;; Used to be in C, but it's pointless since (= 0 n) is faster anyway because