### Define and Use a Simple Actor in Riker

Source: https://riker.rs

This snippet demonstrates how to define a basic actor with a String message type and how to instantiate and send a message to it within an ActorSystem. Ensure the ActorSystem is properly initialized.

```rust
struct MyActor;

impl Actor for MyActor {
    type Msg = String;

    fn receive(&mut self,
                ctx: &Context<Self::Msg>,
                msg: Self::Msg,
                sender: ActorRef<Self::Msg>) {

        println!("received {}", msg);
    }
}

let sys = ActorSystem::new(&model).unwrap();

let props = MyActor::props();
let a = sys.actor_of(props, "a").unwrap();

a.tell("Hello actor!".to_string(), None);

```

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