- review accum, tail-recursion.
  Write sum, as tail-recursive.
  Write foldr.  
    (While we could've used my-foldr and my-foldr-helper,
     that would be fine  but we'll have to pass along 'combine' function
     to the helper, which never changes.)
     Instead of passing it along, we opt to make the helper-function
     a local-constant, using let.
     Oh wait, we need to use letrec.
    )

- Sexpr: data-def'n.  Example: html.
   (see ~/Library/Preferences/.GlobalPreferences.plist.orig)

  To do:
   Code which processes s-exprs.  (Really, same as file and family-trees.)
   Write total-count -- using map, and (apply + ...)
   Write equal? -- using map, and (apply and ...).  Wait, need andmap.