hold :: a -> Event a -> Interval (Behavior a)
hold a e = do
    (b, update) <- newBehavior a
    on e update
    return b
hold :: a -> Event a -> Interval (Behavior a)
hold a e = do
    (b, update) <- newBehavior a
    on e update
    return b
hold a e = do
    (b, update) <- newBehavior a
    on e update
    return b