(rename-file "a" "b") fails if b exists

[Robert Morris <rtm@uun.org>]

If I call (rename-file "a" "b") in clisp-1995-12-08, and file b
exists, clisp raises an error:

*** - RENAME-FILE: File #"/usr/rtm/clisp-1995-12-08/src/b" already exists

And there doesn't seem to be a way to suppress it: no :if-exists :supersede.

CLTL1 doesn't specify that this should be an error. I want rename to
work in this situation so I can update files without risking losing
them if the system crashes. For instance, the following can be
interrupted at any time and real-file will contain either new or old
data, but not just be truncated:

  (with-open-file (s "temporary-file" :direction :outout)
    ...
    (finish-output s))
  (rename-file "temporary-file" "real-file")

But this doesn't work in clisp.

People who want the existing behaviour of rename-file can test if the
target file exists first.