anchor.diff:    Pass either an object of type butcher-tableau or a symbol.
     If passed a symbol, it's associated tableau will
     be retrieved from a pre-defined list."
  -  (let1 (m (if (symbolp tableau) `(cdr (assoc (quote ,tableau) *tableaus*)) tableau))
  +  (let1 (m (if (symbolp tableau)
  +             `(cdr (assoc (quote ,tableau) *tableaus*))
  +             tableau))
       `(let1 (*selected-tableau* ,m)
          (if (any-p *selected-tableau* null (not butcher-tableau-p))
            (error "Not a valid butcher-tableau: ~A~%" *selected-tableau*))
  
positive.code:       (apply #'mapcar (list #'fma x (swap-matrix-layout k))))
      x))
  
  (defun scaled-ode-eval (ode stepsize ai ci k t0 x0)
    (mapcar (lambda (fi) (* stepsize fi))
            (funcall ode
                     (+ t0 (* ci stepsize))
                     (eval-point x0 k ai))))
  
  (defun solve-explicit-rks (ode x0 t0 stepsize)
    (let ((k))
      (loop for ai in *A*
            for ci in *c*
            do (let ((ki (list (scaled-ode-eval ode stepsize ai ci k t0 x0))))
                 (appendf k ki)))
      k))
  
  
negative.code:                (x0 (cadar closest)))
            (if (< t1 t0)
              (error "Can't go back in time."))
            (if (num-equal t0 t1)
              x0
              (let1 (graph (runge-kutta-graph the-ode x0 t0 t1 stepsize))
                (insert-at closest graph)
                (cadar (last graph))))))
        (lambda () known-graph))))
  