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HTML is good for debugging, but is unsuitable for application use.
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To see the non HTML representation of the XML format, set format=xml.
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<?xml version="1.0"?>
<api>
  <query-continue>
    <allcategories accontinue="Década_de_1950" />
  </query-continue>
  <query>
    <allcategories>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">Anuarios</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Archivo Audiovisual del Instituto de Investigaciones Gino Germani</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Biblioteca Enerc</c>
      <c size="38" pages="38" files="0" subcats="0" xml:space="preserve">Biblioteca Nacional</c>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">CEDINCI</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Columnas y suplementos de cine en publicaciones de interés general</c>
      <c size="3" pages="3" files="0" subcats="0" xml:space="preserve">Década de 1910</c>
      <c size="6" pages="6" files="0" subcats="0" xml:space="preserve">Década de 1920</c>
      <c size="27" pages="27" files="0" subcats="0" xml:space="preserve">Década de 1930</c>
      <c size="25" pages="25" files="0" subcats="0" xml:space="preserve">Década de 1940</c>
    </allcategories>
  </query>
</api>