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According to arch-hacker James Gosling, to "...modify a
program with no idea whatsoever how it works." (See
whacker.) It is actually possible to do this in nontrivial
circumstances if the change is small and well-defined and you
are very good at
glarking things from context. As a trivial
example, it is relatively easy to change all "stderr" writes
to "stdout" writes in a piece of C filter code which remains
otherwise mysterious.