anchor.diff:            (arg2 (input-value node 'arg2)))
         (+ arg1 arg2)))
   
  -  (defclass mult-node (computation-node computation-node-mixin source-node-mixin sink-node-mixin)
  +  (defclass mult-node (computation-node computation-node-mixin source-sink-mixin)
       ())
   
     (defmethod compute ((node mult-node))
  
positive.code: 
  #+(or)
  (progn
    (defclass plus-node (computation-node computation-node-mixin source-node-mixin sink-node-mixin)
      ())
  
    (defmethod compute ((node plus-node))
  
negative.code: 
  (defclass source-node-mixin ()
    ((validp :accessor validp :initform nil)
     (sinks :accessor sinks :initform nil))
    (:documentation "Mixin class that provides slots and some methods for the SOURCE-NODE protocol
  class."))
  
  (defmethod (setf input) :after ((new-val source-node-mixin) sink input-name)
    (let ((node-entry (find sink (sinks new-val) :key #'car)))
  