code: static public Key[] exit(Key... keep) {
      List<Key> keylist = new ArrayList<>();
      if( keep != null )
        for( Key k : keep ) if (k != null) keylist.add(k);
      Object[] arrkeep = keylist.toArray();
      Arrays.sort(arrkeep);
      Stack<HashSet<Key>> keys = _scope.get()._keys;
      if (keys.size() > 0) {
        Futures fs = new Futures();
        for (Key key : keys.pop()) {
          int found = Arrays.binarySearch(arrkeep, key);
          if ((arrkeep.length == 0 || found < 0) && key != null) Keyed.remove(key, fs);
        }
        fs.blockForPending();
      }
      return keep;
    }
docs: Exit the inner-most Scope, remove all Keys created since the matching
  enter call except for the listed Keys.
  @return Returns the list of kept keys.