Alternative, alist-based representation for trees in lisp:

((car . define)
 (caadr . flatten->alist)
 (cadadr . lis)
 (caddadr . #!optional)
 (caadddadr . idx)
 (caadadddadr . quote)
 (caadadadddadr . #\r)
 (cdadadadddadr)
 (cddadadddadr)
 (cddadddadr)
 (cddddadr)
 (caaddr . if)
 (caadaddr . pair?)
 (cadadaddr . lis)
 (cddadaddr)
 (caaddaddr . append)
 (caadaddaddr . flatten->alist)
 (caadadaddaddr . car)
 (cadadadaddaddr . lis)
 (cddadadaddaddr)
 (caaddadaddaddr . cons)
 (cadaddadaddaddr . #\a)
 (caddaddadaddaddr . idx)
 (cdddaddadaddaddr)
 (cdddadaddaddr)
 (caaddaddaddr . flatten->alist)
 (caadaddaddaddr . cdr)
 (cadadaddaddaddr . lis)
 (cddadaddaddaddr)
 (caaddaddaddaddr . cons)
 (cadaddaddaddaddr . #\d)
 (caddaddaddaddaddr . idx)
 (cdddaddaddaddaddr)
 (cdddaddaddaddr)
 (cdddaddaddr)
 (caadddaddr . list)
 (caadadddaddr . cons)
 (caadadadddaddr . string->symbol)
 (caadadadadddaddr . list->string)
 (caadadadadadddaddr . cons)
 (cadadadadadadddaddr . #\c)
 (caddadadadadadddaddr . idx)
 (cdddadadadadadddaddr)
 (cddadadadadddaddr)
 (cddadadadddaddr)
 (caddadadddaddr . lis)
 (cdddadadddaddr)
 (cddadddaddr)
 (cddddaddr)
 (cdddr))

Advantages:

* Like alists, purely functional; completely eliminates need for
  set-car!.
* Like alists, fast (constant-time) alteration at any depth within the
  tree.
* Simple textual representation -- fewer nested parentheses.
* Highly flexible representation for source: e.g., indents
  automatically; safely allows sorting lines.

Disadvantages:
* None apparent.