code: def subject_areas(self):
          
          areas = self.xml.findall('subject-areas/subject-area')
          freqs = self.xml.findall('author-profile/classificationgroup/'
                                   'classifications[@type="ASJC"]/classification')
          c = {int(cls.text): int(cls.attrib['frequency']) for cls in freqs}
          cats = [(a.text, c[int(a.get("code"))], a.get("abbrev"), a.get("code"))
                  for a in areas]
          cats.sort(reverse=True, key=itemgetter(1))
          return cats
docs: List of tuples of author subject areas in the form
          (area, frequency, abbreviation, code), where frequency is the
          number of publications in this subject area.