static <A> Map<A> consequentsMap(Collection<A> c) {
  ret consequentsMap(new Map, c);
}

static <A> Map<A> consequentsMap(Map<A> map, Iterable<A> c) {
  A last = null;
  for (A a : unnull(c)) {
    if (last != null && a != null) {
      map.put(last, a);
      last = a;
    }
  }
  ret map;
}