code: def snapshots_to_send(source_snaps, dest_snaps):
      
      if len(source_snaps) == 0:
          raise AssertionError("No snapshots exist locally!")
      if len(dest_snaps) == 0:
          # nothing on the remote side, send everything
          return None, source_snaps[-1]
      last_remote = dest_snaps[-1]
      for snap in reversed(source_snaps):
          if snap == last_remote:
              # found a common snapshot
              return last_remote, source_snaps[-1]
      # sys.stderr.write("source:'{}', dest:'{}'".format(source_snaps, dest_snaps))
      raise AssertionError("Latest snapshot on destination doesn't exist on source!")
docs: return pair of snapshots