code: def _diagnose_prefixes(self):
          
          from peyotl.collections_store import COLLECTION_ID_PATTERN
          p = set()
          for owner_dirname in os.listdir(self.doc_dir):
              example_collection_name = "{n}/xxxxx".format(n=owner_dirname)
              if COLLECTION_ID_PATTERN.match(example_collection_name):
                  p.add(owner_dirname)
          return p
docs: Returns a set of all of the prefixes seen in the main document dir