<item>
  <name>service_agreement_3yr</name>
  <base_cost>150_000</base_cost>
  <!-- bought widgets within the last thirty days, 
       first year is free -->
  <cost_hook>
    <apply_order>100</apply_order>
    <cod><![CDATA[ 
    sub {
      my ( $cart, $item, $customer, $cost ) = @_;
      my $widgets_order = $cart->customer()->last_ordered('widget');
      if ( $widgets_order and 
           $widgets_order->date() > time()-60*60*24*30 ) {
        return 100_000;
      } else {
        die;
      }
    } ]]></cod>
   </cost_hook>
</item>

<customer>
  <email>foo@bar.com</email>
  <cost_hook>
    <!-- 3 orders or more discount -->
    <apply_order>400</apply_order>
    <cod><![CDATA[ 
      Foo::Util::get_Std_3_Orders_hook() 
   ]]></cod></cost_hook>
   <cost_hook>
    <!-- Joe belongs to the 50%-off club -->
    <apply_order>300</apply_order>
    <cod><![CDATA[ sub {  
    sub {
      my ( $cart, $item, $customer, $cost ) = @_;
        return $cost / 2;
      }
    } ]]></cod>
   </cost_hook>
</customer>

#----

<DocumentDefinition>
  <name>item</name>
  <element>widget</element>
  <element><name>base_cost</name></element>
  <nested_element>
    <name>cost_hook</name>
    <element>apply_order</element>
    <element>code</element>
  </nested_element>
  <plural>'cost_hook'</plural>
  <store>
    <name>main</name>
    <base>items</base>
    <location>Read_only_file:'extension','.item'</location>
  </store>
</DocumentDefinition>

<DocumentDefinition>
  <name>customer</name>
  <element><name>email</name></element>
  <nested_element>
    <name>cost_hook</name>
    <defname>item:cost_hook</defname>
  </nested_element>
  <plural>'cost_hook'</plural>
  <method>
    <name>num_orders</name>
    <cod><![CDATA[
      sub { 
        my $self = shift;
        return XML::Comma::Def->read('order')
          ->get_index->('completed')->count 
          ( where_clause => 'doc_id="' . $self->doc_id() . '"' );
    ]]></cod>
  </method>
  <method>
    <name>last_ordered</name>
    <cod><![CDATA[
      sub { 
        my ( $self, $item ) = @_;
        return XML::Comma::Def->read('order')
          ->get_index->('orders')->single
          ( where_clause => 'doc_id="' . $self->doc_id() . '"',
            collection_spec => "items:$item",
            order_by        => "timestamp DESC" );
    ]]></cod>
  </method>
  <store>
    <name>main</name>
    <base>customers</base>
    <location>Sequential_dir</location>
    <index_on_store>main</index_on_store>
  </store>
  <index>
    <name>main</name>
    <field>email</field>
  </index>
</DocumentDefinition>

<DocumentDefinition>
  <name>order</name>
  <element><name>customer_id</name></element>
  <element><name>timestamp</name></element>
  <element><name>total_paid</name></element>
  <nested_element>
    <name>item</name>
    <element>item_name</element>
    <element>item_quantity</element>
    <element>price_paid</element>
  </nested_element>
  <plural>'item'</plural>
  <method>
    <name>number_of</name>
    <cod><![CDATA[
      sub { 
        my ( $self, $item ) = @_;
        my $total_quantity;
        map { 
          $total_quantity += $_->item_quantity() 
            if $_->item_name() eq $item;
        } $self->elements('item')
    ]]></cod>
  </method>
  <method>
    <name>make_totals</name>
    <cod><![[CDATA[
    sub {
      my $self = shift;
      my $total = 0;
      my $customer = XML::Comma::Doc
           ->read('customer|main|'.$self->customer_id()) ||
             die "oops, couldn't get customer doc";
      foreach my $i ( $self->elements('item') ) {
        my $this_total = $i->base_cost();
        foreach $hook
          ( sort { $a->apply_order() <=> $b->apply_order() }
               ( $customer->elements('cost_hook'),

                 $i->elements('cost_hook') ) ) {
            my $cost;
            eval { $this_total = $hook->code()->( $cart, $ ) };
            return $cost unless $@;
        }
        my $extended = $this_total * $i->item_quantity();
        $i->price_paid ( $extended );
        $total += $extended;
      }
      $this->total_paid ( $total );
      return $total;
    }
    ]]></cod>
  <store>
    <name>carts</name>
    <base>carts</base>
    <location>Derived_file: 'derive_from','doc_id',
                            'extension','.order'</location>
  </store>
  <store>
    <name>orders</name>
    <base>orders</base>
    <location>Sequential_file</location>
    <index_on_store>orders</index_on_store>
  </store>
  <index>
    <name>orders</name>
    <field>customer_id</field>
    <field>timestamp</field>
    <field>total_paid</field>
    <collection>
      <name>items</name>
      <cod><![CDATA[ 
        sub { return map { $_->item_name() } $_[0]->item() };
      ]]></cod>
    </collection>
  </index>
</DocumentDefinition>
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(Comma uses the code keyword pretty extensively, which conflicts with the monks-tag of the same name. I shortened Comma's to "cod".)