NIP-107 Relay-to-relay sync

The title says it all.

I'm proposing to implement server-to-server (relay-to-relay) synchronization, so for example when the user posts something in relay ABC, the connnected relay XYZ (with ABC<->XYZ relay-to-relay connection) gets the information about posting in ABC. Then clients which are connected to XYZ relay only (i.e. no connection to realy ABC), get notified that someone they are subscribed to (other client) get posted something on ABC trough the relay XYZ.