nominal parallel = ()
nominal serial = ()

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

union dag result =
    | ParallelNode :: forall result. task parallel result -> dag result

inl parallel_node forall result. (task : task parallel result) : dag result =
    ParallelNode task

inl main () : dag string =
    parallel_node (Publish "not-parallel")