class input_stream :?len:int -> ?block_size:int -> Netchannels.in_obj_channel ->in_obj_stream
in_obj_stream on top of an in_obj_channel. The block_size
 can be specified; it defaults to 4096. 
 If len is passed, this parameter limits the length of the channel:
 Only the first len bytes are read from the input channel, then an EOF
 is simulated even if the input channel is longer.