code: def __check_modalities_to_train(self):
      modalities_to_train = [
          mod for mod in self.modalities if mod not in self.mapping
      ]
      return set(modalities_to_train)
  
file_path: NiftyNet/niftynet/layer/histogram_normalisation.py
imports: from __future__ import absolute_import, print_function, division
  import os
  import numpy
  import tensorflow
  import niftynet.utilities.histogram_standardisation
  from niftynet.layer.base_layer import DataDependentLayer
  from niftynet.layer.binary_masking import BinaryMaskingLayer
label_desc: Use image node_id 3 for calling the HistogramNormalisationLayer obj's underlying member method code with example usage: obj.__check_modalities_to_train() and returns: set