abstract sclass ListEntryWithPrevAndNext<A> implements IVarWithPrevAndNext<A, ListEntryWithPrevAndNext<A>> {
}

static ListEntryWithPrevAndNext<A> listEntryWithPrevAndNext(L<A> l, int idx) {
  if (idx < 0 || idx >= l(l)) null;
  ret new ListEntryWithPrevAndNext<A>() {
    pulic A get() { ret get(l, idx); }
    public void set(A a) { l.put(idx, a); }
    public ListEntryWithPrevAndNext<A> prev() { ret ListEntryWithPrevAndNext(l, idx-1); }
    public ListEntryWithPrevAndNext<A> next() { ret ListEntryWithPrevAndNext(l, idx+1); }
  };
}