new example: ANS PICK and ROLL words

FossilOrigin-Name: ee9dcdbb5edbd8b30e0155ee5cde126254a180236b83ae2bf19d26a3f950befe

RELEASE_NOTES.md

@@ -101,6 +101,7 @@ June 2019.
 ## Examples
 
 - add Abort.forth
+- add ANS-PICK-ROLL.forth
 - add atua-gophermap.forth
 - add Buffer.forth
 - add CaptureOutput.forth

example/ANS-PICK-ROLL.forth

@@ -0,0 +1,57 @@
+PICK and ROLL are problematic in that they require the ability
+to address the stack as if it were an array. The implementations
+here are not efficient as RETRO's stacks are *not* addressable.
+
+These will never be added to the standard image, but are provided
+here as an aid in porting ANS FORTH code or for those curious as
+to how such things could be added.
+
+# PICK
+
+6.2.2030 PICK 
+CORE EXT
+
+	( xu ... x1 x0 u -- xu ... x1 x0 xu )
+
+Remove u. Copy the xu to the top of the stack. An ambiguous
+condition exists if there are less than u+2 items on the stack
+before PICK is executed.
+
+
+~~~
+{{
+  :save-stack (...-a)
+    here [ depth &, times ] dip ;
+  :fetch-prior (a-n[a-1])
+    dup fetch swap n:dec ;
+  :restore-stack (a-...)
+    here swap - here n:dec swap [ fetch-prior ] times drop ;
+---reveal---
+  :PICK (...n-...m)
+    &Heap [ [ save-stack ] dip
+            over + fetch [ restore-stack ] dip ] v:preserve ;
+}}
+~~~
+
+# ROLL
+
+6.2.2150 ROLL 
+CORE EXT 
+
+	( xu xu-1 ... x0 u -- xu-1 ... x0 xu )
+
+Remove u. Rotate u+1 items on the top of the stack. An ambiguous
+condition exists if there are less than u+2 items on the stack
+before ROLL is executed.
+
+~~~
+{{
+  :save-values (...n-a)
+    [ , ] times here ;
+  :restore-values (a-...)
+    here - here swap [ fetch-next swap ] times drop ;
+---reveal---
+  :ROLL (...n-...m)
+    &Heap [ save-values ] v:preserve swap [ restore-values ] dip ;
+}}
+~~~