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.