static <A> LPair<A> overlappingPairs_lazy(L<A> l) {
  if (l == null) null;
  
  class OverlappingPairs extends LazyList<Pair<A>> {
    final int n = l(l)+1;
    
    public int size() { ret n; }
    
    public Pair<A> get(int i) {
      ret pair(l.get(i), l.get(i+1));
    }
  }
  
  ret new OverlappingPairs;
}