Substitute environment variables referred to in FILENAME.
`$FOO' where FOO is an environment variable name means to substitute
the value of that variable. The variable name should be terminated
with a character not a letter, digit or underscore; otherwise, enclose
the entire variable name in braces.
If `/~' appears, all of FILENAME through that `/' is discarded.
If `//' appears, everything up to and including the first of
those `/' is discarded.
(fn FILENAME)