datatype Direction = req | ack
Value = {0..3}
channel wrt : Direction.Value
channel rd : Direction.Value

Cell = 
  let CellState(val) =
    rd.req?dumb -> rd.ack!val -> CellState(val) [] wrt.req?x -> wrt.ack.x -> CellState(x)
  within
    CellState(0)