Extending dict (dict's) to allow for multidimensional dictionary

[Ravi]

I can extend dictionary to allow for the my own special look-up tables. However now I want to be able to define multidimensional dictionary which supports look-up like this:

d[1]['abc'][40] = 'dummy'

and if d[1] and d[1][abc] raise KeyError just create them.

for d[1] I can override __getitem__() but how to do the same for the d[1][abc]?