Function instance_key_param 

pub fn instance_key_param() -> &'static str

The module-level binding a component’s state field lives in. Prefixed with the component’s name so two components’ n are two different bindings. The parameter every instance-aware synthesized function carries: WHICH instance it is acting for. A component’s state is a map from this key to that instance’s value, so two placements of one component keep separate records, each instance addressed by its key path.

The ROOT instance’s key is the empty string, so a component placed once — every page that worked before — is the general case with a constant key rather than a second mechanism.