Issue: PATHNAME-SUBDIRECTORY-LIST (Version 4)

[alarson@src.honeywell.com (Aaron Larson)]

    Allow the value of a pathname's directory component to be a list.  The
    car of the list may be either of the symbols :ABSOLUTE or :RELATIVE.
    Each remaining element of the list is a string or one of the keyword
    symbols listed below...

I picked on Kent for not specifying that the elements of the list should be
either strings or keywords, then after reading PATHNAME-WILD, I think that
we should not preclude implmentations from defining "regular expression",
or "user home directory"components.  E.g. 

  (pathname-directory x) => (:absolute "foo" #<regexp "Fo" :wild "bar">)
  (pathname-directory x) => (#<user-homedir "alarson"> "bar" "baz")

I'm not advocating adding such a feature, just not precluding us from
defining one in the future.  Perhaps we should add a line something like:

  "Implementations may permit objects of types other than keywords and
  strings as elements of the pathname-directory list." 

Even without a statement of this kind, I support the proposal.