Wonka Rules Engine : Intermediate Example : VAT Calculation (Biz Rules)

<?xml version="1.0"?>
<RuleTree xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

   <if description="Start a new sales transaction">
      <criteria op="AND">
         <eval id="pop1">(N.NewSalesTransSeq) POPULATED</eval>
      </criteria>

      <if description="Checking Input Values">
           <criteria op="AND">
               <eval id="pop2">(N.NewSaleEAN) POPULATED</eval>
               <!-- Since Solidity does not support decimals, we must divide in order to get a close (but still imprecise) estimate -->
               <eval id="asn0">(N.NewSaleVATRateDenom) INVOKE_VAT_LOOKUP (N.NewSaleItemType, N.CountryOfSale)</eval>
           </criteria>

           <validate err="severe">
               <criteria op="AND">
                   <eval id="cmp2">(N.NewSalePrice) GT (0.00)</eval>
                   <eval id="cmp3">(N.PrevSellTaxAmount) GT (0.00)</eval>
               </criteria>

               <failure_message>ERROR!  Required inputs for VAT calculation have not been provided.</failure_message>
               <success_message/>
           </validate>           
       </if>

      <if description="Execution of the VAT Calculation">
           <criteria op="AND">
               <eval id="cmp4">(N.NewSaleVATRateDenom) GT (0.00)</eval>
               <eval id="cmp5">(N.PrevSellTaxAmount) GT (0.00)</eval>
           </criteria>

           <validate err="severe">
               <criteria op="AND">
                   <eval id="asn1">(N.NewSellTaxAmount) ASSIGN_QUOT (N.NewSalePrice, N.NewSaleVATRateDenom)</eval>
                   <eval id="asn2">(N.NewVATAmountForHMRC) ASSIGN_DIFF (N.NewSellTaxAmount, N.PrevSellTaxAmount)</eval>
               </criteria>

               <failure_message>ERROR!  Unable to assign the figures related to the VAT.</failure_message>
               <success_message/>
           </validate>           
       </if>        

   </if>    
    
</RuleTree>