Ah, I see. You mean (X)HTML, and 'a' is P, 'b' is SPAN, 'c' is CODE, 'd' is SMALL, 'e' is EM and 'f' is STRONG. So, now you encounter:

  <P>foo <SPAN> bar  baz <EM> qux </EM> <EM> quux </EM> </P>
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Now, assuming tags aren't to be inserted inside words, I still can find five places to put in </SPAN>: before 'bar', between 'bar' and 'baz', after 'baz', between the two EM elements, and before the </P> tag.

Now, if you have a DTD that says that the only possible content of a 'b' is exactly two 'e's, you know where the missing closing tag should have been.

Note also that if you have a DTD where you can always unambigiously deduce where a missing closing tag should have gone, the closing tag is redundant - and if it were an SGML DTD instead of an XML DTD, the closing tag would have been optional. (And that would have solved the problem instantly - the document would be conforming).