##
12.9.2 `MultiFile` Example

      fp = MultiFile(sys.stdin, 0)
      fp.push(outer_boundary)
      message1 = fp.readlines()
      # We should now be either at real EOF or stopped on a message
      # boundary. Re-enable the outer boundary.
      fp.next()
      # Read another message with the same delimiter
      message2 = fp.readlines()
      # Re-enable that delimiter again
      fp.next()
      # Now look for a message subpart with a different boundary
      fp.push(inner_boundary)
      sub_header = fp.readlines()
      # If no exception has been thrown, we're looking at the start of
      # the message subpart.  Reset and grab the subpart
      fp.next()
      sub_body = fp.readlines()
      # Got it.  Now pop the inner boundary to re-enable the outer one.
      fp.pop()
      # Read to next outer boundary
      message3 = fp.readlines()