code: def add_filter(self, table, cols, condition):
          
          if table is not None and table not in self.relations:
              raise ItsdbError('Cannot add filter; table "{}" is not defined '
                               'by the relations file.'
                               .format(table))
          # this is a hack, though perhaps well-motivated
          if cols is None:
              cols = [None]
          self.filters[table].append((cols, condition))
docs: Add a filter. When reading *table*, rows in *table* will be
          filtered by filter_rows().
  
          Args:
              table: The table the filter applies to.
              cols: The columns in *table* to filter on.
              condition: The filter function.