anchor.diff:       finally (return t)))
   
   (defun check-backward-position-nil (tree bitvector)
  +  (declare (type simple-bit-vector bitvector))
     (loop for end = nil then x
        for x = (position 0 bitvector :from-end t :end end)
        while x
  
positive.code:      finally (return t)))
  
  (defun check-forward-iteration (tree bitvector)
    (loop with iter = (trees::make-iterator tree :forwardp t)
       for start = 0 then (1+ x)
       for x = (position 1 bitvector :start start)
  
negative.code:      finally (return t)))
  
  (defun check-backward-position-t (tree bitvector)
    (declare (type simple-bit-vector bitvector))
    (loop for end = nil then x
       for x = (position 1 bitvector :from-end t :end end)
       for i downfrom (1- (trees:size tree))
  