Safe and sound STRef

[oleg@pobox.com]

> Heaps should be more dynamic than this; the (type of the) *reference*
> should encode the type it points to, but the (type of the) *heap*
> should not.

However, the heap can store polymorphic values. Therefore, we can use
a heap to store the polymorphic heap... Your example, slightly
re-written as follows, types

test5 = do
	let heapi = init_gh
	let (href,heap2) = alloc_gh undefined heapi
	let (mr,heap1) = 
	       if 1<2 then
                   let (xr,h) = alloc_gh 42 heapi 
	               heap2' = alter_gh href h heap2
	           in (Just xr,heap2')
               else
                   (Nothing,heap2)
 	print $
 	  case mr of
              Nothing -> ""
              Just r  -> show (fetch_gh r (fetch_gh href heap1))

and even prints the answer to everything...