tell(X)

If X is an open output stream, then it is made the current output stream.

If X is an atom, it is taken to be a file name and
   - if there is an output stream currently associated with a file
     of that name then it is made the current output stream.( If
     more than one output stream is associated with X, one is
     chosen arbitrarily)
   - otherwise, that file is opened, a stream created and and set
     to be the current output stream
