sub ranges_beth2 {
  my ($M, $N) = @_;
  my $r = ($M+1)%$N;
  my $q = ($M+1-$r)/$N;
  my $iEnd=0;
  return [ map { my $iStart=$iEnd; [$iStart, ($iEnd+=$_) - 1]
               } (($q+1) x $r, ($q) x ($N-$r)) ];
}