anchor.diff:  
   (defun merge-tables! (table &rest tables)
     (reduce (lambda (x y)
  +            (check-same-test x y)
               (maphash
                (lambda (k v)
                  (setf (gethash k x) v))
  
positive.code: 
  (defconstant +hash-table-default-size+
    (hash-table-size (make-hash-table)))
  
  (defun copy-hash-table/empty
      (table
  
negative.code:   "Merge TABLE and TABLES, working from left to right.
  The resulting hash table has the same parameters as TABLE.
  
  If the same key is present in two tables, the value from the rightmost
  table is used.
  
  All of the tables being merged must have the same value for
  `hash-table-test'.
  
  Clojure's `merge'.
  "
    (let ((size (max +hash-table-default-size+
                     (reduce #'+ tables
                             :key #'hash-table-count
  