The following two subdirectories approach the problem of modifying
memory accesses for "special" addresses.  Details are in file mem.lisp
in each directory (see especially comments near the top), but in
summary:

- mem-access-sound/ uses wrappers for reading and writing memory,
  which bypass the primitive read and write operations when given a
  special address; and

- mem-access-unsound/ modifies the behavior of stobj primitives at
  special addresses, in a way that is unsound.
