in src/spark_automerge_wrapper.js [194:204]
function processSyncMessage(state, syncStates, sourcePeerId, syncMessage) {
  const [nextBackend, nextSyncState, patch] = Automerge.receiveSyncMessage(
    state,
    syncStates[sourcePeerId] || initSyncState(),
    syncMessage
  )
  syncStates[sourcePeerId] = nextSyncState
  return [nextBackend, patch]
}