static long[] concatLongArrays(L<long[]> arrays) {
  int l = 0;
  for (long[] a : unnull(arrays)) l += l(a);
  long[] x = new long[l];
  int i = 0;
  for (long[] a : unnull(arrays)) if (a != null) {
    System.arraycopy(a, 0, x, i, l(a));
    i += l(a);
  }
  ret x;
}

static long[] concatLongArrays(long[]... arrays) {
  ret concatLongArrays(asList(arrays));
}

Began life as a copy of #1026923