pub fn offer_choice<C, Row1, Row2, SessionSum, InjectSessionSum>(
cont1: impl FnOnce(InjectSessionSum) -> SessionSum + Send + 'static
) -> PartialSession<C, ExternalChoice<Row1>> where
C: Context,
Row1: Send + 'static,
Row1: ToRow<Row = Row2>,
Row2: RowCon,
Row2: ElimSum,
Row2: SplitRow,
Row2: SumFunctor,
Row2: SumFunctorInject,
Row2: SumApp<SessionF<C>, Applied = SessionSum>,
Row2: FlattenSumApp<InjectSessionF<Row1, C>, FlattenApplied = InjectSessionSum>,
SessionSum: Send + 'static,
InjectSessionSum: Send + 'static,