e("/", get(outer_handler))
    .merge(inner_router)
    .with_state(OuterState {});
# let _: axum::Router = app;
```

# Merging routers with fallbacks

When combining [`Router`]s with this method, the [fallback](Router::fallback) is also merged.
However only one of the routers can have a fallback.

# Panics

- If two routers that each have a [fallback](Router::fallback) are merged. This
  is because `Router` only allows a single fallback.