code: def write(self, obj, **kwargs):
          
          super().write(obj, **kwargs)
          for name, ss in obj.items():
              key = 'sparse_series_{name}'.format(name=name)
              if key not in self.group._v_children:
                  node = self._handle.create_group(self.group, key)
              else:
                  node = getattr(self.group, key)
              s = SparseSeriesFixed(self.parent, node)
              s.write(ss)
          self.attrs.default_fill_value = obj.default_fill_value
          self.attrs.default_kind = obj.default_kind
          self.write_index('columns', obj.columns)
docs: write it as a collection of individual sparse series