return a
print(merge({(1): {'a': 'A'}, (2): {'b': 'B'}}, {(2): {'c': 'C'}, (3): {'d':
    'D'}}))
merge({(1): {'a': 'A'}, (2): {'b': 'B'}}, {(1): {'a': 'A'}, (2): {'b': 'C'}})
Dictionaries of dictionaries merge