+val readChar : bio -> char option
+val readInt : bio -> int option
+val readLen : bio * int -> string option
+val readChunk : bio -> string option
+val readString : bio -> string option
+
+val writeChar : bio * char -> unit
+val writeInt : bio * int -> unit
+val writeString' : bio * string -> unit
+val writeString : bio * string -> unit