create a variable xsl

[tin]

Good morning, if anyone would know it was helping to create a variable through a xsl and xmlI have this xml:\[code\]<!ELEMENT bdticketCardell (invoice, services, products) >........<!ELEMENT services (service+)><!ELEMENT service (nameService, quantityService, priceService)><!ELEMENT nameService (#PCDATA) ><!ELEMENT quantityService (#PCDATA) ><!ELEMENT priceService (#PCDATA) ><!ATTLIST service idService ID #REQUIRED>\[/code\]I want to do= ""variable name="grosspriceservice1" select="//quantityService[@idService = "s1"] * //priceService[@idService = "s1"]/>""Many thanks