The comparison A is B may succeed when A is an element,
text, comment, or PI node, and B is an attribute or
namespace node, if the two nodes happen to have the
same offset in their respective data structures.
This bug has been present since the introduction of the
TinyTree, it is also present in 6.5.3. (Although the
"is" operator is not available in XSLT 1.0, the same
code is used internally for evaluating a union and for
Reported by Anton Lapounov [firstname.lastname@example.org]
Circumvention: reverse the test (use B is A).
Test case added: expr90
Source code fixed.