docs: def stack(self, dimensions=None, **dimensions_kwargs):
          """
          Stack any number of existing dimensions into a single new dimension.
  
          New dimensions will be added at the end, and the order of the data
          along each new dimension will be in contiguous (C) order.
  
          Parameters
          ----------
          dimensions : Mapping of form new_name=(dim1, dim2, ...)
              Names of new dimensions, and the existing dimensions that they
              replace.
          **dimensions_kwargs:
              The keyword arguments form of ``dimensions``.
              One of dimensions or dimensions_kwargs must be provided.
  
          Returns
          -------
          stacked : Variable
              Variable with the same attributes but stacked data.
  
          See also
          --------
          Variable.unstack
          """
          dimensions = either_dict_or_kwargs(dimensions, dimensions_kwargs,
                                             'stack')
          result = self
          for new_dim, dims in dimensions.items():
              result = result._stack_once(dims, new_dim)
          return result