My vision just now was to create a sand box with a DSL (that doesn't use floats) and have others write the rules into it.

   VAT.include(country, type, amount){
     if(type == "high" && country == "NL"){
       return percentageOf(price, VAT.NL.high) + amount;
     }
     throw new Error("Error in VAT.include: VAT not found."); 
   }