anchor.diff:           for c = (sqrt (+ (* x x) (* y y)))
            collect (list (vector x y) (vector (if (> c diameter) 0 1))))))
        
  +(defun xor-training ()
  +  (list (list #(0.0 0.0) #(1.0))
  +        (list #(0.0 1.0) #(0.0))
  +        (list #(1.0 0.0) #(0.0))
  +        (list #(1.0 1.0) #(1.0))))
  
positive.code:        (set-layer-transfer-function net (layer-index layer) transfer-tag)))
  
  (defmethod transfer ((neuron t-neuron))
    (setf (output neuron)
          (funcall (transfer-function neuron) (input neuron)))
    neuron)
  
negative.code: 
  (defmethod initialize-instance :after ((neuron t-neuron) &key)
    (setf (id neuron) (incf (next-id (net neuron))))
    (when (biased neuron)
      (setf (input neuron) 1.0))
    (setf (transfer-function neuron)
          (transfer-functions (transfer-tag neuron) :function))
    (setf (transfer-derivative neuron)
  