anchor.diff:  
   (defmacro with-weights-copied ((from-bpn) &body body)
     "In BODY ->WEIGHT will first look up if a weight lump of the same
  -name exists in FROM-BPN and return that, or else create a weight lump
  -normally. If FROM-BPN is NIL, then weights are copied."
  +  name exists in FROM-BPN and return that, or else create a weight
  +  lump normally. If FROM-BPN is NIL, then weights are copied."
     (alexandria:with-gensyms (%from-bpn)
       `(let* ((,%from-bpn ,from-bpn)
               (*lumps-to-copy*
  
positive.code: 
  (defun add-lump (lump bpn)
    "Add LUMP to BPN. MAX-N-STRIPES of LUMP gets set to equal that of
  the previous last, non-weight lump of BPN."
    (when (find-lump (name lump) bpn)
      (error "Cannot add ~S: ~%
              a lump of same name has already been added to this network." lump))
  
negative.code: 
  (defmethod cost (bpn)
    "Return the sum of costs for all active stripes. The cost of a
    stripe is the sum of the error nodes. The second value is the number
    of stripes."
    (let ((sum (flt 0)))
      (loop for lump across (lumps bpn) do
        (when (typep lump '->error)
  