Suppose this data:

<foo:bar xmlns:foo="http://test/ns/foo" a="b">Yadda</foo:bar>

Then suppose this stylesheet:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
 B B B  xmlns:foo="http://test/ns/foo"
 B B B  version="2.0">

 B B B  <xsl:template match="foo:bar">
 B B B B B B B  <xsl:if test="ancestor-or-self::foo*[@a]">

This is
  ancestor-or-self::foo * [@a]
that is ancestor-or-self::foo multiplied with the array [@a], i.e. the
array constructed with the attribute "a", I think.

Not sure what the multiplication with an array should result into, need
to scan the XPath 3.1 spec in some detail.