... conditionally swaps one string with another ...
EVERYWHERE ..., using a host of options, such as whole-word
search, comment-/string-exclusion, interactive
verification, and more, to ensure you don't shoot yourself
in the foot. It can even produce a mass-verification
preview for you to be certain that the changes are
just/only what you really intended before committing them.
(Used heavily by Sweeney and Optimus.)