I thought it would work.... but the generated id is different each time the
 document is loaded ! What is an id good for if it changes everytime ?

the id is unique within a run of the stylesheet so it can be used in
xslt code to identify a node as in <xsl:if
test="generate-id(.)=generate-id($xx)">
or it can be used to generate ids and internal links in teh output so
long as you don't mind that they are unstable and change if document
regenerated.

To get stable ids either get them from the source or use something like
<xsl:number/> to generate a unique id.
ie <xsl:attribute name="id">foo<xsl:number level="any"
count="foo"/></xslattribute"/>

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