my $genome_iter = i_map(
  sub {join '', @_},
  join_iter(
    map {
      comb_iter(
        map {
          list_iter(qw(a c g n t));
        } 1..$_
      )
    } 2..3
  )
);

while (my $string = $genome_iter->()) {
  print "$string\n";
}



# Takes a list of iterators, and returns an iterator that iterates
# over each in turn
sub join_iter {
  my @iter = @_;
  my $i = 0;
  return sub {
    while ($i < @iter) {
      my @ret = $iter[$i]->();
      if (@ret) {
        return wantarray ? @ret : $ret[0];
      }
      else {
        $i++;
      }
    }
    $i = 0;
    return ();
  };
}