Execution example::

  select WebClips --output_columns "html_untag(content)" --command_version 2
  # [
  #   [
  #     0, 
  #     1337566253.89858, 
  #     0.000355720520019531
  #   ], 
  #   [
  #     [
  #       [
  #         2
  #       ], 
  #       [
  #         [
  #           "html_untag", 
  #           null
  #         ]
  #       ], 
  #       [
  #         "groonga is fast"
  #       ], 
  #       [
  #         "mroonga is fast"
  #       ]
  #     ]
  #   ]
  # ]
