anchor.diff:         (if (any-p *selected-tableau* null (not butcher-tableau-p))
            (error "Not a valid butcher-tableau: ~A~%" *selected-tableau*))
          (let ((*A* ) (*b*) (*c*) (*is-implicit*))
  -         (update-variables *selected-tableau*)
  +         (update-selected-tableau *selected-tableau*)
            ,@body))))
   
   (defun eval-point (x k aj)
  
positive.code:           *c* time-coeffs
            *is-implicit* is-implicit)))
  
  (update-variables (make-butcher))
  
  (defmacro with-tableau (tableau &body body)
    "Create context that defines a specific butcher tableau.
  
negative.code: ;define known tableau names as keywords
  (mapcar #'make-keyword (mapcar #'string (mapcar #'first *tableaus*)))
  
  (defvar *selected-tableau* (make-butcher))
  (defvar *A*)
  (defvar *b*)
  (defvar *c*)
  (defvar *is-implicit*)
  
  (defun update-selected-tableau (selected-tableau)
    (declare (type butcher-tableau selected-tableau))
    (with-slots (matrix weights time-coeffs is-implicit) selected-tableau
      (setf *A* matrix
  