static <A, B> L<B> lambdaMapLike lazyMap_bitSet(IF1<A, B> f, L<A> l) {
  ret lazyMap_bitSet((O) f, l);
}

static L lazyMap_bitSet(fO f, fL l) {
  ret new RandomAccessAbstractList {
    final int size = l(l);
    new BitSet bitSet;
    new L data;
    
    public int size() { ret size; }
    public O get(int i) {
      if (bitSet.get(i))
        ret data.get(i);
      bitSet.set(i);
      O o = callF(f, l.get(i));
      listPut(data, i, o);
      ret o;
    }
  };
}

Began life as a copy of #1018394