class output_filter :io_obj_channel -> out_obj_channel ->out_obj_channel
output_filter filters the data written to it through the
io_obj_channel (usually a pipe), and writes the filtered data
to the passed out_obj_channel.
If the filter is closed, the io_obj_channel will be closed, too,
but not the destination out_obj_channel (so you can still append
further data).
