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 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.