(cl-fad:walk-directory dirname fn &key directories (if-does-not-exist error)
(test (constantly t)))
|Function: Recursively applies the function FN to all files within the directory named by the non-wild pathname designator DIRNAME and all of its sub-directories. FN will only be applied to files for which the function TEST returns a true value. If DIRECTORIES is not NIL, FN and TEST are applied to directories as well. If DIRECTORIES is :DEPTH-FIRST, FN will be applied to the directory's contents first. If DIRECTORIES is :BREADTH-FIRST and TEST returns NIL, the directory's content will be skipped. IF-DOES-NOT-EXIST must be one of :ERROR or :IGNORE where :ERROR means that an error will be signaled if the directory DIRNAME does not exist.