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                                         871
SECTION 10.6                                                           Logical Structure



   3 0 obj                             % Page's content stream
       << /Length … >>
   stream
       …
       /Fm4 Do                         % Paint form XObject
       …
   endstream
   endobj

   4 0 obj                             % Form XObject
       << /Type /XObject
          /Subtype /Form
          /Length …
          /StructParent 6              % Parent tree key
       >>
   stream
       …
   endstream
   endobj

   100 0 obj                           % Parent tree (accessed from structure tree root)
      << /Nums [ 0 101 0 R
                 1 102 0 R
                 …
                 6 10R                 % Entry for page object 2; points back
                 …                     % to parent structure element
               ]
      >>
   endobj

For a content item that is a marked-content sequence, the retrieval method is
similar but slightly more complicated. Because a marked-content sequence is not
an object in its own right, its parent tree key is found in the StructParents entry of
the page object or other content stream in which the sequence resides. The value
retrieved from the parent tree is not a reference to the parent structure element
itself but to an array of such references—one for each marked-content sequence
contained within that content stream. The parent structure element for the given
sequence is found by using the sequence’s marked-content identifier as an index
into this array (see Example 10.13).

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