def _test():
    """

    >>> a = NonZeroDimensionalComponent(dimension = 1, free_variables='x')
    >>> b = NonZeroDimensionalComponent(dimension = 1, free_variables='y')
    >>> c = {'z' : 4}
    >>> d = {'z' : 3}
    >>> e = ZeroDimensionalComponent([c, d])
    >>> f = utilities.MethodMappingList([a,b,e])
    >>> f.flatten()
    [{'z': 4}, {'z': 3}]
    >>> f.keys()[:-1]
    [NonZeroDimensionalComponent(dimension = 1, free_variables = 'x'), NonZeroDimensionalComponent(dimension = 1, free_variables = 'y')]
    >>> f.keys()[2].dimension
    0
    >>> set.union(*[set(keys) for keys in f.keys().flatten()]) == set('z')
    True
    """
    pass
