code: def trim_columns(self, columns_to_trim):
          
          # TODO check if trimmed column is actually one of the columns
          if len(self._trimmed_columns) == 0:
              self._trimmed_columns.append(columns_to_trim)
          else:
              self._trimmed_columns.extend(columns_to_trim)
          self._trimmed_columns = self._trimmed_columns[0]
          self.encoder['trimmed_columns'] = self._trimmed_columns
docs: remove column in design matrix