Note on unique activity IDs

> [!TIP]

> The activity's `id` does not have to be necessarily dereferenceable—it's

> totally fine to use a URL with a fragment identifier, such as

> `https://example.com/123#follow`.

> `https://example.com/123#follow/751d477f-2167-4473-ace5-4404f4760c0d`.

> [!CAUTION]

> Sometimes you may tempted to derive the activity's `id` from the actor and

> the object's `id` properties, but keep in mind that the same kind of

> activities can be made by the same actor to the same object multiple times.

>

> For example, if Alice sends a `Follow` activity to Bob, Bob accepts it,

> Alice sends an `Undo(Follow)` activity to Bob, and Alice sends a `Follow`

> activity to Bob again, the `id` of the `Follow` activity should be different

> from the previous one, even if the `actor` and the `object` are the same.

>

> If they have the same `id`, the recipient may mistakenly think that the

> activity is a duplicate of the previous one and ignore it, which is not

> what you want.

>

> To ensure the uniqueness of the activity's `id`, it is recommended to contain

> a UUID or a similar unique identifier in the `id` property.

The most of cases, an activity should have the `to` property, which corresponds

to the recipient of the activity.  If it's a public activity, you can set