Elm problem to Route Cmds to Subs

CMDtoSUB.elm

{-| This sketch should show the problem I have.

I have 2 Modules. The first Module should "receive"
Messages from another module. I could create a large and complex Msg model,
OR I could use Cmd and Sub (I think).

So my Idea is :
    Second.Trigger -> Cmd "Send Start" -> Sub "Listen on Start" -> First.Start
                                       ^------ this is the part which unclear to me

I don't want to use ports for that!

-}



-- | the module that should receive messages from Module Second
module First exposing (..)

type Msg = Start | Stop

type alias Model = { started : Bool }

update : Msg -> Model -> Model
update msg model = case msg of
                    Start ->  { started = True }
                    Stop -> { started = False }

subscriptions : Model -> Sub msg
subscriptions _ = ListenOnStart Start

ListenOnStart : ??
ListenOnStart = ??



-- | the module that should send messages to Module First
module Second exposing (..)

type Msg = Trigger

type alias Model = {}

update : Msg -> Model -> (Model, Cmd msg)
update Trigger = ( {}, TriggerListenOnStart )

TriggerListenOnStart : ??
TriggerListenOnStart = ??