Associates an output file with the current buffer.

  This command associates an output file to a buffer for writing the
  buffer out to a file.  This command does not cause the buffer to be
  written to a file.  You can disassociate the current buffer from an
  output file by specifying this command with a blank output file name.
  In this case, LSE asks you to supply a file name when you request
  that the buffer be written out.