code: def field_xml_output(field, tag):
      
      marcxml = []
      if field[3]:
          marcxml.append('  <controlfield tag="%s">%s</controlfield>' %
                         (tag, MathMLParser.html_to_text(field[3])))
      else:
          marcxml.append('  <datafield tag="%s" ind1="%s" ind2="%s">' %
                         (tag, field[1], field[2]))
          marcxml += [_subfield_xml_output(subfield) for subfield in field[0]]
          marcxml.append('  </datafield>')
      return '\n'.join(marcxml)
docs: Generate the XML for field 'field' and returns it as a string.