docs: def transform(self, X):
          """
          Delete all features, which were not relevant in the fit phase.
  
          :param X: data sample with all features, which will be reduced to only those that are relevant
          :type X: pandas.DataSeries or numpy.array
  
          :return: same data sample as X, but with only the relevant features
          :rtype: pandas.DataFrame or numpy.array
          """
          if self.relevant_features is None:
              raise RuntimeError("You have to call fit before.")
  
          if isinstance(X, pd.DataFrame):
              return X.copy().loc[:, self.relevant_features]
          else:
              return X[:, self.relevant_features]