anchor.diff:            *c* time-coeffs
             *is-implicit* is-implicit)))
   
  -(update-variables (make-butcher))
  +(update-selected-tableau (make-butcher))
   
   (defmacro with-tableau (tableau &body body)
     "Create context that defines a specific butcher tableau.
  
positive.code: ;define known tableau names as keywords
  (mapcar #'make-keyword (mapcar #'string (mapcar #'first *tableaus*)))
  
  (defvar *selected-tableau*)
  (defvar *A*)
  (defvar *b*)
  (defvar *c*)
  (defvar *is-implicit*)
  
  (defun update-variables (selected-tableau)
    (declare (type butcher-tableau selected-tableau))
    (with-slots (matrix weights time-coeffs is-implicit) selected-tableau
      (setf *A* matrix
  
negative.code:        (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-selected-tableau *selected-tableau*)
           ,@body))))
  
  (defun eval-point (x k aj)
  