sub FETCH {
   my ($self, $i) = @_;
   return $self->_fetch($i);
}

sub fetch {
   my ($self, $i) = @_;
   $i += $self->num_eles() if $i < 0;
   return undef if $i < 0;
   return $self->_fetch($i);
}

sub _fetch {
   my ($self, $i) = @_;
   # $i is guaranteed to be in range
   ...
}