#! /usr/local/bin/perl -w

use strict;


my @nuc = qw/ A T C G /;

sub inc {
        $_[0] eq 'A' and return 'T';
        $_[0] eq 'T' and return 'C';
        return 'G';
}

sub gen {
    my @current = @_;
    my $done = 0;
    return sub {
         return if $done;
         my @res = @current;
         my $i;
         ITER: for ($i = 0; $i < scalar @current; ++$i) {
             if( $current[$i] ne $nuc[-1] ) {
                 $current[$i] = inc($current[$i]);
                 last;
             }
             else {
                 $current[$i] = $nuc[0];
             }
         }
         $done = 1 if $i >= scalar @res;
         return @res;
    }
}

my @set = ('A') x (shift || 3);
my $iter = gen( @set );

while( my @iter = $iter->() ) {
    print "@iter\n";
}