sub concatenate {
  my $self = shift;
  my ($new_seq,$prepend) = @_;
  croak "argument to concatenate must be a string or a Sequence object"
    if ref($new_seq) && !$new_seq->isa(__PACKAGE__);
  my $to_append = ref($new_seq) ? $new_seq->seq : $new_seq;

  return $self->new($prepend ? $to_append . $self->seq 
                     : $self->seq . $to_append);
}

Example 2: The concatenate() method.