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11-Apr-86 21:15:24 -George Carrette

Problems: To have a global lexical environment.

          To be able to have multiple Macsyma listeners with
          a different state in each.


(defun foo (exp)
   (funcall #'bar exp)
   (if (eq (car exp) 'baz)
       '(bar)
      ....))

The problem is that there are so many cells which can can have side effects
made on them.

Lemma: New data type INDEX-REF-POINTER. 
       [<INDEX-REF-POINTER><OFFSET>]

       Similar to [<EXTERNAL-VALUE-CELL-POINTER><ADDRESS>]
       Which means fetch location at <ADDRESS> and look at that.

       More similar to [<SELF-REF-POINTER><OFFSET>]
       Which means fetch <OFFSET> of current value of SELF.

    
       [<INDEX-REF-POINTER><OFFSET>] means fetch the location
       at (PLUS <A-INDEX-REF-POINTER-BASE>  <OFFSET>) and look at that.

Lemma: Primitives for allocating environments and binding the A-INDEX-REF-POINTER-BASE.

Solutions: 
 [1] In each cell that is subject to a side effect while running the application, e.g.
   - value cells.
   - function cells.
   - property lists.
  => put an INDEX-REF-POINTER.

 [2] A global lexical environment may be had by using the INDEX-REF-POINTER in
   the FEF locations (such as used for function cell and special variable references)
   normally containing EXTERNAL-VALUE-CELL-POINTER. 
   -> would need to modify FASLOAD to allocate these.

 * For Macsyma. Could use [1]. Could even decide after loading it that you want multiple
   copies. Could implement forking of sessions. Could also use [2] on the function
   cell and value cell and [1] for the property list. Although it may be better to
   shadow PUTPROP and GET et al and implement our own plist mechanism, perhaps using
   a hash table. Significant data compactness advantage over existing system
   performance could perhaps be had this way.


Stinger:
  One may well want to enclose over <A-INDEX-REF-POINTER-BASE>.