List.rotor: redesign second argument from overlap to gap

explicit allow negative gaps to mean overlap

S32-setting-library/Containers.pod

@@ -683,14 +683,16 @@ The functional form assumes it is working on a list of integers C<^$n>.
 
 =item rotor
 
-    multi method rotor ( $elems = 2, $overlap = 1 )
+    multi method rotor ( Int() $elems, Int() $gap = 0 )
 
 Returns a new list of sublists formed by returning a sublist of C<$elems>
-elements, backing up by C<$overlap> elements, then repeating.  Stops as
+elements, advancing/skipping C<$gap> elements, then repeating.  Stops as
 soon as there are no longer enough remaining elements to form a sublist
 that is C<$elems> long.  (From which it follows that an infinite list
 can be rotorized forever.)
 
+Negative gaps are allowed, meaning the result lists will contain overlap.
+
 =item pairup
 
  multi method pairup ( )