nominal parallel = ()
nominal serial = ()

union task effect result =
    | ValidateIssuer :: task parallel string
    | Publish :: string -> task serial string

union rec dag result =
    | ParallelNode :: forall result. task parallel result -> dag result
    | SerialNode :: forall result. task serial result -> dag result
    | Then :: forall result.
        (exists input. dag input * (input -> dag result))
        -> dag result

inl main () : dag string =
    Then (exists (
        ParallelNode ValidateIssuer,
        fun (total : i32) => SerialNode (Publish "invalid")
    ))