anchor.diff:  
   ;;;; Scratch ------------------------------------------------------------------
   
  -#+no (-<> scully.zdd::*rules*
  +(-<> '(
  +       (ggp-rules::<= a (ggp-rules::true foo))
  +       (ggp-rules::<= b (ggp-rules::true foo) (ggp-rules::true baz))
  +       (ggp-rules::<= c (ggp-rules::true dogs) (ggp-rules::not a))
  +       (ggp-rules::<= x (ggp-rules::not a) b)
  +       (ggp-rules::<= y (ggp-rules::not a) (ggp-rules::not x))
  +       (ggp-rules::<= y (ggp-rules::not c))
  +       )
     (integerize-rules <>)
  -  ; (never <>)
  +  (nth 2 <>)
  +  (gethash :possible <>)
  +  (stratify-layer <>)
  +  (mapc (lambda (s) (format t "Stratum:~%~{    ~S~%~}" s)) <>)
  +  (never <>)
     ; (map nil #'print-hash-table <>)
     )
  
positive.code:               layers))))
  
  
  ;;;; API ----------------------------------------------------------------------
  (defun integerize-term (term->number term)
    (match term
  
negative.code:                       (mark-dependency head (bare-term b))))))
        (iterate (for rule :in rules)
                 (for (head . body) = (ensure-list rule))
                 (digraph:insert-vertex graph head)
                 (mark-dependencies head body)))
      graph))
  
  