code: def __len__(self):
      return sum(len(dataset) for dataset in self._datasets)
  
file_path: chainer/chainer/dataset/tabular/_concat.py
imports: import six
  from chainer.dataset.tabular import tabular_dataset
label_desc: Use image node_id 2 for calling the _Concat obj's underlying member method code with example usage: obj.__len__() and returns: sum