static int falsesFollowedByTrues_changeIndex_assertNotNull(L<Bool> l) {
  int i = 0, n = l(l);
  while (i < n && isFalse(l.get(i))) ++i;
  for (int j = i; j < n; j++)
    if (!isTrue(l.get(i)))
      fail("Not a false/true list: " + l);
  ret i;
}