Rename fields x to y and y to x

    Parameters
    ----------
    obj : dict_like
        Object with labels to rename

def flip_labels(obj):
    """
    Rename fields x to y and y to x

    Parameters
    ----------
    obj : dict_like
        Object with labels to rename
    """
    def sub(a, b):
        """
        Substitute all keys that start with a to b
        """
        for label in list(obj.keys()):
            if label.startswith(a):
                new_label = b+label[1:]
                obj[new_label] = obj.pop(label)

    sub('x', 'z')
    sub('y', 'x')
    sub('z', 'y')
    return obj