First the code fragment...
<tag name="default" abc="10" def="20> <!-- not valid, abc and def should be mutually exclusive -->
<tag name="default1" abc="10"> <!-- valid -->
<tag name="default2" def="20> <!-- valid -->
<tag name="default<tag name="default" abc="10" def="20> <!-- not valid, abc and def should be mutually exclusive -->
<tag name="default1" abc="10"> <!-- valid -->
<tag name="default2" def="20> <!-- valid -->
<tag name="default