code: def get_truth_joint_feature_vector(self, idx):
      return self.make_psi(self.samples[idx], self.labels[idx])
  
file_path: dlib/python_examples/svm_struct.py
imports: import dlib
label_desc: Use image node_id 3 for calling the ThreeClassClassifierProblem obj's underlying member method code with example usage: obj.get_truth_joint_feature_vector(idx) and returns: self