- What took me time:
  Either cut/pasting and forgetting to change the name fo the struct;
  or, going back and adding lines to a function and forgeting
    to put in the recursive calls for *sub*structures
    (resulting in the wrong type getting passed arond)
- Bug: when evaling an appl, thought I didn't need to eval the appl-fun,
  since it would already be substituted.
  However, for applying curried functions, it is necessary.



- an intentional (if confusing?) name mis-match:
   the struct 'sum' for the syntax 'plus'.
- I highly advise writing out
  some test cases by hand, for function-application.




Scheme version:
- the expr->string is rally bogus; leave it out?
- the diff between { and (.
- parsing practically solved for us, with 'read'




Java version:
- no good way to test return value of 'fun' ?
- parsing more hands-on.
  (Consider writing a scanner method equivalent to scheme's 'read'?)