code: def add_applicator(self, table, cols, function):
          
  
          if table not in self.relations:
              raise ItsdbError('Cannot add applicator; table "{}" is not '
                               'defined by the relations file.'
                               .format(table))
          if cols is None:
              raise ItsdbError('Cannot add applicator; columns not specified.')
          fields = set(f.name for f in self.relations[table])
          for col in cols:
              if col not in fields:
                  raise ItsdbError('Cannot add applicator; column "{}" not '
                                   'defined by the relations file.'
                                   .format(col))
          self.applicators[table].append((cols, function))
docs: Add an applicator. When reading *table*, rows in *table* will be
          modified by apply_rows().
  
          Args:
              table: The table to apply the function to.
              cols: The columns in *table* to apply the function on.
              function: The applicator function.