Function component 

pub fn component<S, V, St, I, Step, Out>(
    initial: I,
    step: Step,
    output: Out,
) -> ErasedComponent<S, V>
where
    S: Send + Sync + 'static,
    V: 'static,
    St: Send + 'static,
    I: Fn() -> St + Send + Sync + 'static,
    Step: Fn(St, &Fact) -> St + Send + Sync + 'static,
    Out: Fn(&St) -> (V, Vec<Transition<S>>) + Send + Sync + 'static,