sub mutate() {
        my $self = shift;
        # my $strength = shift || 2;
        my($i, $p1, $p2, $c1, $c2);

        my $my_astr = $self->astr();   

        # Get $strength (2) positions and flip the characters
        $c1 = substr($my_astr, $p1 = int rand(length($my_astr)), 1);
        $c2 = substr($my_astr, $p2 = int rand(length($my_astr)), 1);

        substr($my_astr, $p1, 1, $c2);
        substr($my_astr, $p2, 1, $c1);
        
        return $self->astr($my_astr);
}