static class DerivedMap<A, B> extends AbstractMap<A, B> {
  Map<A, B> base;
  Map<A, B> additions = new HashMap;
  
  *() {}
  *(Map<A, B> *base) {}
  *(Map<A, B> *base, Map<A, B> *additions) {}
  
  public B get(Object key) {
    B b = additions.get(key);
    if (b != null) return b;
    return base.get(key);
  }
  
  public B put(A key, B value) {
    return additions.put(key, value);
  }
  
  public Set<Map.Entry<A,B>> entrySet() {
    throw fail();
  }
}

Began life as a copy of #1001027