class input_channel :?onclose:unit -> unit -> Pervasives.in_channel ->in_obj_channel
in_channel, which must be open.
 The method pos_in reflects the real position in the channel as
 returned by Pervasives.pos_in. This works for both seekable and
 non-seekable channels.
 The method close_in also closes the underlying in_channel.
 The function onclose is called after the in_channel has been closed.