non-lifo blocks

[Craig Chambers]

To enforce non-LIFO-ness of blocks, you'd have to check every time you stored
an argument into a local var or a variable in the heap.  This would undoubtedly
be much more overhead than checking when a scope that has blocks returns.  The
compiler is pretty smart about optimizing this overhead anyway.  My thesis 
(out Real Soon Now (tm)) shows that block zapping has a negligible cost (< 1%)
in the current system.

-- Craig Chambers