static <A> LPair<IntRange, A> streaksOfRepeatingElements(L<A> l) {
  new LPair<IntRange, A> out;
  int i = 0, n = l(l);
  while (i < n) {
    A a = l.get(i);
    int j = i+1;
    while (j < n && eq(a, l.get(j))) ++j;
    out.add(pair(intRange(i, j), a));
    i = j;
  }
  ret out;
}