It's a bit hard to explain question in plain English or plain code, I
try to provide simple
XML
<MyRoot>
<LovelyTag1>
  <LovelyTag2>
   <Item/>
  </LovelyTag2>
</LovelyTag1>

<LovelyTag1>
<BadTag1>
  <LovelyTag2>
   <Item/>
  </LovelyTag2>
</BadTag1>
</LovelyTag1>
</MyRoot>

Is there any way to evaluate query:
"I want to get all items with only LovelyTag1 and LovelyTag2 as
ancestors (and myroot of course) but items with any parents except
LovelyTag1 and 2 should be discarded".
And I have very random BadTag's collection, but I can strictly define
LovelyTag's one.
all tags can be follow in random order.

I have only one idea how to do this - serialize ancestor axis into
string, then for each in LovelyTag collection do replacement in
ancestor string every LovelyTag Entry with "/" (or empty string) for
example. And if after all replacements our ancestor string will contain
only "/" 's - this is our Item. But if there any other symbol\tag - bad
item. I know that this is ugly solution but I have no ideas how to
check against not defined tag collection.

Is there any other way to solve such task ?