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.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