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Probe-File and proposed extension.

    Should Probe-File never signal an error, even if it is given a
namestring which is malformed?

    There needs to be some way to determine whether a file name is
legal for output.  I brought this issue up once before on this mailing
list, but I think the discussion died.  I am bringing it up again
because we need something of the sort in Spice Lisp, so I am going to
implement it.  My proposed solution is the following:  Probe-File
should take a direction argument, which defaults to :input.  The
semantics of Probe-File would be to return the pathname which would be
used if the file was opened in that direction, and to return NIL if
the open would fail.  Secondary query: Should the direction argument
be a keyword or an optional?  I favor an optional, since I can't think
of any other arguments that want to be added to Probe-File.