anchor.diff:      :when (and pt (< pt ot)) :do (setf ot pt)
       :finally (return ot)))
   
  +;; XXX I am suspicious of this code since if you pass it a pathname to
  +;; a file which doesn't exist, the newest timestamp is the most in the
  +;; past timestamp which could exist.
   (defun newest-timestamp (pathnames)
     (loop :with nt = most-negative-single-float
       :for p :in pathnames
  
positive.code:                                    :collect (grain-pathname-text env o)))))
  
      ;; first, check the outputs are newew than the inputs, if so, bail.
      (when (and (all-newer-pathnames-p output-pathnames input-pathnames) ; same checking as with make
  	       (not force)) ; force computation
        (return-from run-computation t))
      ;; create the output directories
      (dolist (output-pn output-pathnames)
        (ensure-directories-exist output-pn))
      (dolist (external-command (external-commands-for-computation env command))
        (log-format 5 "running:~{ ~A~}" (mapcar #'escape-shell-token external-command))
        (let ((text (run-program/read-output-string external-command)))
  
negative.code:     (&rest keys &key
       source-registry setup verbosity output-path
       build lisp-implementation lisp-binary-path define-feature undefine-feature
       disable-cfasl master object-directory use-base-image profiling debugging
       force)
    (declare (ignore source-registry setup verbosity
                     lisp-implementation lisp-binary-path define-feature
  		   undefine-feature disable-cfasl master object-directory
  		   use-base-image debugging))
    (with-maybe-profiling (profiling)
      (apply 'handle-global-options keys)
      (simple-build build :output-path output-path :force force)))
  