How to maintain keys in TransformDict?

[oldman1991]

I see the demo:
'''
>>> d = TransformDict(str.lower)
>>> d['Foo'] = 5
>>> d['foo'] == d['FOO'] == d['Foo'] == 5
True
>>> set(d.keys())
{'Foo'}
'''
but , i dont know the object how to maintain the keys.
thanks