Protecting CONS cells from modification

[Telford Tendys <telford@eng.uts.edu.au>]

Just writing down a thought as it comes to me...

If a given block of cons cells get scanned during
a garbage collection cycle, at the end of GC you
could take a CRC value for that block of CONS cells
and store it out of the way somewhere.

First thing next GC, you take a CRC of the same
block and if it is different there must have been
a set-cdr! or similar somewhere so you need to rescan,
if not then you can safely assume that the block
have not been touched.

OK, I know that it is vaguely possible that multiple
bits in the block of cells could change and result in
identical CRC values but that is very unlikely.
Thus, you get yet another method of checking for modification
to CONS cells which is not as intrusive as SCM_ASSIGN
and faster and more portable than using the MMU chip.

It won't work if any of those CONS cells are SMOBs or
vectors either, can't win em all!

	- Tel