[MLton] ref flattening enabled

[Stephen Weeks]

> I don't remember with the boxing stuff if a box-ref was needed to
> get at the value in the box.  I am assuming that it would be needed
> (and would like it to be required).  I.e., an 'a box is just like an
> 'a ref except that it isn't mutable. 

Right.

> (I guess also the box `constructor' is non-expansive, unlike the ref
> `constructor'.)

Hmmm.  I had thought to make it a primitive type and a pair of
functions (box, unbox), but non-expansiveness is a good reason to make
it a constructor.