XMI tags with XSLT loops

I'm finding a problem with the xmi tags ( example : UML:Package) my real problem is that I can't use for-each loop with the select option (select="UML:Package") . Here's the declaration of the namespace :

<xsl:stylesheet version="2.1"
        xmlns:xsl="http://ift.tt/tCZ8VR" xmlns:xmi="http://ift.tt/1PWHVNc"
        xmlns:UML="org.omg.xmi.namespace.UML">

any help?