Non-obvious XPath: Getting elements that do not have a particular attribute or child element

Well sounds pretty simple.. Here's an example. Consider the following source XML document

<?xml version="1.0"?>
<Wizards>
<Wizard Name="Merlin" Level="5">
 <Spells>
   <Spell Name="SkinOfOil"/>
 </Spells>
</Wizard>
<Wizard Name="Apprentice">
 <Spells/>
</Wizard>
</Wizards>

So now to select all Wizards that do not have a Level attribute...

/Wizards/Wizard[not(@Level)]

Similarly to select all Wizards that do not know any Spells...

/Wizards/Wizard[not(Spells/Spell)]

The key here is the not function which reduces the argument to a boolean value. Once again XPath amazes me with its simplicity