According to arch-hacker James Gosling (designer of [NeWS], [GOSMACS] and Java), 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 [glark]ing 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.
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