matrix-doc/api/server-server/examples/pdu_v4.json

{
    "$ref": "pdu_v3.json",
    "auth_events": [
        "$urlsafe_base64_encoded_eventid",
        "$a-different-event-id"
    ],
    "prev_events": [
        "$urlsafe_base64_encoded_eventid",
        "$a-different-event-id"
    ],
    "redacts": "$some-old_event"
}