static <A> Map<A, MultiSet<Int>> runLengthStats(L<A> list) {
  Map<A, MultiSet<Int>> map = new Map;
  int n = l(list), i = 0;
  while (i < n) {
    A a = list.get(i);
    int j = i+1;
    while (j < n && eq(a, list.get(j))) ++j;
    MultiSet<Int> ms = getOrCreate(map, a, -> new MultiSet);
    ms.add(j-i);
    i = j;
  }
  ret map;
}