whack
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 {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.
[{Jargon File}]