code: def length(cls, dataset):
          
          if not dataset.data:
              return 0
          length = 0
          ds = cls._inner_dataset_template(dataset)
          for d in dataset.data:
              ds.data = d
              length += ds.interface.length(ds)
          return length+len(dataset.data)-1
docs: Returns the length of the multi-tabular dataset making it appear
          like a single array of concatenated subpaths separated by NaN
          values.