def _validate_singletons(self, boxes):
count = self._collect_box_count(boxes)<EOL>multiples = [box_id for box_id, bcount in count.items() if bcount > <NUM_LIT:1>]<EOL>if '<STR_LIT>' in multiples:<EOL><INDENT>raise IOError('<STR_LIT>')<EOL><DEDENT>
Several boxes can only occur once.