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Input: <thead> ... <row><col>abc</col><col>bcd</col><col>def</col></row> <row><col>abc</col><col>bcd</col><col>def</col></row> </thead>
Input: <thead> ... <row><col>abc</col><col><ex>bcd</ex></col><col>def</col></row> <row><col>abc</col><col>bcd</col><col>def</col></row> </thead>