anchor.diff:       finally (return t)))
   
   (defun check-selection (tree bitvector)
  +  (declare (type simple-bit-vector bitvector))
     (unless (zerop (trees:size tree))
       (loop for start = 0 then (1+ x)
          for x = (position 1 bitvector :start start)
  
positive.code:        (return t)))
  
  (defun check-reduce-from-end (tree bitvector)
    (loop with tree-diff = (trees:reduce tree #'- :initial-value 0 :from-end t)
       for start = 0 then (1+ x)
       for x = (position 1 bitvector :start start)
  
negative.code:     t))
  
  (defun check-reduce (tree bitvector)
    (declare (type simple-bit-vector bitvector))
    (loop with tree-sum = (trees:reduce tree #'+)
       for start = 0 then (1+ x)
       for x = (position 1 bitvector :start start)
  