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.