Modify: insert-nth to support negative indies

1 files changed, 5 insertions, 1 deletions

diff --git a/utilities.lisp b/utilities.lisp
index 972df8a..214189f 100644
--- a/utilities.lisp
+++ b/utilities.lisp
@@ -38,7 +38,11 @@
   "Creates a new list, the result of inserting X into the Nth position
    of LIST, displacing the rest of the elements by one position. If N
    is greater than the length of LIST, X becomes the last element of
-   LIST.  If N is negative, a list containing just X is returned."
+   LIST.  If N is negative, the element is inserted from the back of
+   the list. If the abslute value of -N is greater than the lenght of
+   the list, a list just containing X is returned."
+  (when (minusp n)
+    (setf n (+ 1 n (length list))))
   (multiple-value-bind (front back) (take n list)
     ;; NCONC ok b/c this call to TAKE returns values that do not shre
     ;; memory with LIST