my ($self, $tmp) = @_;
  if ($tmp) {
    $self->{'ensembl_id'} = $tmp;
  }
  return $self->{'ensembl_id'};

  my ($self, $tmp) = @_;
  if ($tmp) {
    $self->{'external_id'} = $tmp;
  }
  return $self->{'external_id'};

  my $self = shift;
  
  my ($in_id, $out_id, $in_name, $out_cs);
  if ( $_[4] eq $self->{'external'} ) {
    $in_id   = 'external_id';
    $in_name = 'external';
    $out_id  = $self->{'ensembl_id'};
    $out_cs  = $self->{'ensembl_cs'};
  } elsif ( $_[4] eq $self->{'ensembl'} ) {
    $in_id   = 'ensembl_id';
    $in_name = 'ensembl';
    $out_id  = $self->{'external_id'};
    $out_cs  = $self->{'external_cs'};
  } else {
    throw($_[4].' is neither the xref or peptide coordinate system');
  }
  
  my @coords = $self->SUPER::map_coordinates( $in_id, @_[1..3], $in_name );
  for my $c ( @coords ) {
    if ($c->isa('Bio::EnsEMBL::Mapper::Coordinate')) {
      $c->id( $out_id );
      $c->coord_system( $out_cs ); # IdentityXref mapper doesn't set this
    }
  }
  return @coords;
}

1;

  my ( $proto, $from, $to, $from_cs, $to_cs, $identity_xref, $translation ) = @_;
  
  my $is_forward = 1;
  if ($identity_xref->isa('Bio::EnsEMBL::Translation')) {
    $is_forward = 0;
    ($identity_xref, $translation) = ($translation, $identity_xref);
  }
  $identity_xref->can('get_mapper') || throw('Xref does not support mapping');
  
  my $class = ref $proto || $proto;
  my $mapper = $identity_xref->get_mapper;
  my $self = {
    %{ $mapper }
  };
  bless $self, $class;
  
  $self->{'from'}    = $from;
  $self->{'to'}      = $to;
  $self->{'from_cs'} = $from_cs;
  $self->{'to_cs'}   = $to_cs;
  
  $self->{'external'}    = $is_forward ? $from : $to;
  $self->{'ensembl'}     = $is_forward ? $to   : $from;
  $self->{'external_cs'} = $is_forward ? $from_cs : $to_cs;
  $self->{'ensembl_cs'}  = $is_forward ? $to_cs   : $from_cs;
  
  return $self;