Executes before a string search is applied to the index,
returning a new search string.
Variables:
- subs: The string being searched. Be careful about shadowing:
- this variable is actually a method, so use a temporary variable
- or explicitly call self.subs; the substitutions in index.rb
- don't actually work.
+ subs: The string being searched.
EOS
## these two accessors should ONLY be used by single-threaded programs.