Returns a set of all of the prefixes seen in the main document dir

def _diagnose_prefixes(self):
        """Returns a set of all of the prefixes seen in the main document dir
        """
        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