Return a mapping of all MultiIndex levels and their corresponding
        coordinate name.

def _level_coords(self):
        """Return a mapping of all MultiIndex levels and their corresponding
        coordinate name.
        """
        level_coords = OrderedDict()
        for cname, var in self._coords.items():
            if var.ndim == 1 and isinstance(var, IndexVariable):
                level_names = var.level_names
                if level_names is not None:
                    dim, = var.dims
                    level_coords.update({lname: dim for lname in level_names})
        return level_coords