# Dereference so that they can't go behind our backs
 sub fields { return @{ $self->{_fields} } }

 sub add_field {
     my ( $self, $field ) = @_;
     push @{ $self->{_fields} }, $field;
     $self->{_fields_hash}->{ $field } = 
                       scalar @{ $self->{_fields} } - 1;
 }

 sub remove_field {
     my ( $self, $field ) = @_;
     my $idx = $self->field_exists( $field );
     return unless ( $idx );
     delete $self->{_fields_hash}->{ $field };
     splice( @{ $self->{_$fields} }, $idx, 1 );
 }

 sub field_exists {
     my ( $self, $field ) = @_;
     return $self->{_fields_hash}->{ $field };
 }