static class CombinedMap<A, B> extends AbstractMap<A, B> {
  new L<Map<A, B>> maps;

  *() {}
  *(Map<A, B>... maps) { addAllNonNulls(this.maps, maps); }
  <C extends Map<A, B>> *(Collection<C> maps) { addAllNonNulls(this.maps, maps); }

  public int size() { ret lengthLevel2_maps(maps); }
  public Set<Map.Entry<A, B>> entrySet() { fail(); }
  public bool containsKey(O o) {
    for (Map<A, B> map : maps)
      if (map.containsKey(o))
        true;
    false;
  }
  
  @Override
  public B get(O o) {
    for (Map<A, B> map : maps)
      if (map.containsKey(o))
        ret map.get(o);
    null;
  }
}

Began life as a copy of #1021317