Is id always mandatory for bodies and targets? In example 19 seems not.

[giacomomarchioro]

Hi all,
I don't understand if id is mandatory for bodies and targets in:

3.2.1 External Web Resources

id : The IRI that identifies the Body or Target resource.
Bodies or Targets which are External Web Resources must have exactly 1 id with the value of the resource's IRI.

However in the example 19, there is no ID in both body and target body and target are without id:

EXAMPLE 19: Selectors

{
  "@context": "http://www.w3.org/ns/anno.jsonld",
  "id": "http://example.org/anno19",
  "type": "Annotation",
  "body": {
    "source": "http://example.org/page1",
    "selector": "http://example.org/paraselector1"
  },
  "target": {
    "source": "http://example.com/dataset1",
    "selector": "http://example.org/dataselector1"
  }
}

Thanks a lot for your help.