er(obj, '/foo/yet another prop', 'added prop') ==     {'foo': {'another prop': {'baz': 'A string'}, 'yet another prop': 'added prop', 'anArray': [{'prop': 55}]}}
    True

    >>> obj = {'foo': {}}
    >>> set_pointer(obj, '/foo/a%20b', 'x') ==     {'foo': {'a%20b': 'x' }}
    True
    )