code: def findUnique(self, tableClass, comparison=None, default=_noItem):
          
          results = list(self.query(tableClass, comparison, limit=2))
          lr = len(results)
  
          if lr == 0:
              if default is _noItem:
                  raise errors.ItemNotFound(comparison)
              else:
                  return default
          elif lr == 2:
              raise errors.DuplicateUniqueItem(comparison, results)
          elif lr == 1:
              return results[0]
          else:
              raise AssertionError("limit=2 database query returned 3+ results: ",
                                   comparison, results)
docs: Find an Item in the database which should be unique.  If it is found,
          return it.  If it is not found, return 'default' if it was passed,
          otherwise raise L{errors.ItemNotFound}.  If more than one item is
          found, raise L{errors.DuplicateUniqueItem}.
  
          @param comparison: implementor of L{iaxiom.IComparison}.
  
          @param default: value to use if the item is not found.