Makes file F the current output stream.
- If there is an open output stream associated with F
and that was opened previously
by tell/1, then that stream is made the current output
- Otherwise, the specified file is opened for output and made the
current output stream. If the file does not exist, it is created.
Also note that different file names (that is, names which do not unify)
represent different output streams (even if these different file names
correspond to the same file).
- File F does not have write permission, or is a directory.
- F is uninstantiated.