sub i_produce {
  my ($start, $count) = @_;
  for my $i ($start..$start+$count) {
    yield $i;
  };
};

sub i_two_columns {
  while (defined (my $i = produce)) {
    print $i;
    if (defined (my $j = produce)) {
      print "\t$j";
    };
    print "\n";
  };
};