Description:
------------
A few E_NOTICE errors result when using the PDF format, and this may 
reveal other issues. Debugging is as follows:

Reproduce code:
---------------
Debugging simply added this to the notXPath() method definition:

  if ($depth == 0) {
    var_dump(array('tag' => $tag, 'stack' => $this->STACK));
  }

 The problematic line #88::

  if (isset($tag[$this->STACK[--$depth]])) {

Results:

water:phpdoc philip$ phd --format pdf --theme phpbigpdf
[03:15:32 - VERBOSE_INDEXING] Indexing...
[03:15:52 - VERBOSE_INDEXING] Indexing done
[03:15:52 - VERBOSE_FORMAT_RENDERING] Starting pdf rendering
[03:15:52 - VERBOSE_THEME_RENDERING] Using the php theme (phpbigpdf)
[03:15:52 - VERBOSE_RENDER_STYLE] Running full build
array(2) {
  ["tag"]=>
  array(2) {
    [0]=>
    string(15) "format_root_set"
    ["set"]=>
    string(10) "format_set"
  }
  ["stack"]=>
  array(1) {
    [0]=>
    string(3) "set"
  }
}
[03:15:57 - E_NOTICE] /phd/include/PhDReader.class.php:88
	Undefined offset: -1
array(2) {
  ["tag"]=>
  array(2) {
    [0]=>
    string(14) "format_setname"
    ["set"]=>
    bool(false)
  }
  ["stack"]=>
  array(2) {
    [0]=>
    string(3) "set"
    [1]=>
    string(5) "title"
  }
}
[03:15:57 - E_NOTICE] /phd/include/PhDReader.class.php:88
	Undefined offset: -1
array(2) {
  ["tag"]=>
  array(2) {
    [0]=>
    string(15) "format_root_set"
    ["set"]=>
    string(10) "format_set"
  }
  ["stack"]=>
  array(22) {
    [0]=>
    string(3) "set"
    [1]=>
    string(4) "book"
<snip>

[03:17:23 - E_NOTICE] /phd/include/PhDReader.class.php:88
	Undefined offset: -1
[03:17:23 - VERBOSE_TOC_WRITING] Resolving Internal Links... (PHP Manual)
[03:17:25 - VERBOSE_TOC_WRITING] Writing Full PDF Manual (PHP Manual)
[03:17:31 - VERBOSE_FORMAT_RENDERING] Finished rendering
water:phpdoc philip$