<xsl:when test="$counter&lt;'1'">

'1' is the string 1, on which you can't do a < test.
You want the number 1, so

  <xsl:when test="$counter &lt; 1">

> What I am trying to do is simply to test if the number of  ELT is less
> than 1

If that's all you want to do with the count, you don't need a variable
at all:

  <xsl:when test="count(ELT) = 0">

since it's a count frrom a node list, it can't be negative, so = 0
seems a simpler test than < 1.

In fact that is just testing whether there are any ELT, for which you
don't need count()  as an empty node list counts as false,

  <xsl:when test="ELT">


David


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