( ROLL     rotate the n-th stack value to the top of the stack )

need pick
: roll                            ( n --- n )
  1+ dup pick                     ( copy n-th stack value)
  swap 2 * sp@ swap - sp@ swap    ( limits to close gap)
  do
    i 2+ @ i !                    ( move stack values down)
  2 +loop                         ( loop addresses by 2)
  drop                            ( clean up stack)
;