[MLton] space safety for structures

[Stephen Weeks]

> Hmm...  one should ask a real guru, but I supsect if you use CM rather than 
> the REPL you may get a different result.

True.  It works correctly with all the code in one file and even with
A and F in different files.  Although I really can't tell if this is
due to correct handling of space safety or some other optimization
(e.g. cross-module inlining).  Anyways, whether or not SML/NJ does it
right, you've given me a reasonable mental model for how they might.
Thanks.