Function transaction::then

pub fn then<Ctx, A, F, B, Tx2>(a: A, f: F) -> Then<A::Tx, F, Tx2> where
    A: IntoTransaction<Ctx>,
    Tx2: IntoTransaction<Ctx, Item = B, Err = A::Err>,
    F: Fn(Result<A::Item, A::Err>) -> Tx2,