// you should not overflow in the sum...
static long longAverage(long... a) {
  long sum = 0;
  for i over a:
    sum += a[i];
  ret ldiv_round(sum, l(a));
}

static long longAverage(Iterable<Long> l) {
  long sum = 0;
  long n = 0;
  fOr (Long i : l)
    if (i != null) { sum += i; ++n; }
  ret ldiv_round(sum, n);
}

Began life as a copy of #1010188