code: function hookSync(rs) {
    if (rs._dropboxOrigSync) { return; } // already hooked
    rs._dropboxOrigSync = rs.sync.sync.bind(rs.sync);
    rs.sync.sync = function () {
      return this.dropbox.fetchDelta.apply(this.dropbox, arguments).
        then(rs._dropboxOrigSync, function (err) {
          rs._emit('error', new Sync.SyncError(err));
          rs._emit('sync-done');
        });
    }.bind(rs);
  }
docs: Hooking the sync
  
  TODO: document