code: def _diagnose_prefixes(self):
          
          from peyotl.phylesystem import STUDY_ID_PATTERN
          p = set()
          for name in os.listdir(self.doc_dir):
              if STUDY_ID_PATTERN.match(name):
                  p.add(name[:3])
          return p
docs: Returns a set of all of the prefixes seen in the main document dir