How do I test if a combination of keys is in my multi_key_dict

Hi, 

how do I test if a combination of keys is in my multi_key_dict

e.g:

``` python
matrix = multi_key_dict()
matrix['1', '2'] = []

#my try ???

print(('1','2') in matrix)
print(matrix.has_key((key1,key2)))
```

How do I do this ? Or is this possible ?
Thanks
