anchor.diff:      (list (null collection))
       (hash-table (zerop (hash-table-count collection)))))
   
  -(defun empty-lr1-collection (collection)
  +(defun clear-lr1-collection (collection)
     (declare (type lr1-collection collection))
     (typecase collection
       (list '())
  
positive.code:     result))
  
  (defun compute-kernels (grammar)
    (declare (type grammar grammar))
    (let ((p0 (car (grammar-productions grammar))))
      (assert (= 1 (length (production-derives p0))))
  
negative.code:   (action (required-argument) :type simple-vector))
  
  (defun find-precedence (op precedence)
    "Return the tail of PRECEDENCE starting with the element containing OP.
  PRECEDENCE is a list of elements of the form (KEYWORD . (op...))."
    (cond
      ((null precedence) '())
      ((member op (cdar precedence)) precedence)
  