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.