global Io
   optype Read (res char buf[1:*], int count) returns int  num_read
   optype Write (char buf[1:*], int count) returns int  num_written
   optype Seek (int kind, int offset)
   optype Close ()
   type IoDesc = rec(cap Read read; cap Write write; cap Seek seek;
      cap Close close)
   type Access_Type = enum(RD, WR, RW)
end