Manually parsing char-tables

[Richard Wordingham <richard.wordingham@ntlworld.com>]

I am trying to understand how Arabic script rendering works in Emacs
28.0.90, as it seems to be using a different mechanism to that used for
Indic or European scripts.  (There seems to be more to it than just the
asymmetries between right-to-left and left-to-right.)  To that end, I
am trying to understand the contents of the variable
composition-function-table.

When I use command describe-variable, the value shown starts out:

#^[nil nil nil nil

  #^^[1 0

    #^^[2 0 nil nil nil nil nil nil

      #^^[3 768 #1=(["\\c.\\c^+" 1 compose-gstring-for-graphic] 

                    [nil 0 compose-gstring-for-graphic])

        #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1#
        #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1#
        #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1#
        #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1#
        #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1#
        #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1#
        #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# nil
        nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil]

      nil nil 

      #^^[3 1152 nil nil nil #1# #1# #1# #1# #1# #1# #1# nil nil nil
      nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil
      nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil
      nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil
      nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil
      nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil
      nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil
      nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil
      nil nil nil]

(I've converted lines to paragraphs and abbreviated leading white
space.)

 I'm guessing that #1# is a macro invocation; when I invoke (print
 composition-function-table), I get something similar, but with #1#
 expanded and the '#1=' in the apparent macro definition omitted.

Where is this syntax explained?  I've looked in the elisp manual, but
not found it, though I may simply have failed to guess where such a
description was.

Richard.