Path::iterator::rule iterates over files and directories to identify ones matching a user-defined set of rules. the api is based heavily on file::find::rule, but with more explicit distinction between matching rules and options that influence how directories are searched. a path::iterator::rule object is a collection of rules (match criteria) with methods to add additional criteria. options that control directory traversal are given as arguments to the method that generates an iterator.

here is a summary of features for comparison to other file finding modules:
* provides many "helper" methods for specifying rules
* offers (lazy) iterator and flattened list interfaces
* custom rules implemented with callbacks
* breadth-first (default) or pre- or post-order depth-first searching
* follows symlinks (by default, but can be disabled)
* directories visited only once (no infinite loop; can be disabled)
* doesn't chdir during operation
* provides an api for extensions
as a convenience, the pir module is an empty subclass of this one that is less arduous to type for one-liners.