{
    my( $n, $k, @seq, @a );
    sub db {
        my( $t, $p ) = @_;
        if( $t > $n ) {
            push @seq, @a[ 1 .. $p ] if ( $n % $p ) == 0;
        }
        else {
            $a[ $t ] = $a[ $t - $p ];
            db( $t+1, $p );
            for my $j ( $a[ $t - $p ] + 1 .. $k ) { ## Ought to be $k-1, but that doesn't work!
                $a[ $t ] = $j;
                db( $t+1, $t );
            }
        }
    }
    sub deBruijn {
    #    de Bruijn sequence for alphabet k
    #    and subsequences of length n.
        undef @seq; undef @a;
        ( $k, $n ) = @_;
        my @alphabet = split( '', $k );
        $k = $#alphabet;

        @a = (0) x ( $k * $n );
        db( 1, 1 );
        return join( '', @alphabet[ @seq ] );
    }
}