nominal draft = ()
nominal validated = ()
nominal authorized = ()
nominal idempotent = ()

union operation from to effect =
    | Validate :: operation draft validated idempotent

union row from to effect =
    | Row :: forall from to effect. string -> row from to effect

union typed_node from to =
    | TypedNode :: forall from to.
        (exists effect. operation from to effect * row from to effect)
        -> typed_node from to

inl main () : typed_node draft validated =
    inl wrong_row : row validated authorized idempotent = Row "wrong-transition"
    TypedNode (exists (Validate, wrong_row))