Python——找出内层字典中所用到的全部键，并以集合的形式返回

'''
3. 编写一个叫做db_headings的函数，要求能够找出内层字典中所用到的全部键，并以集合的形式返回。
以给出的字典为例，该函数应该返回集合{'author', 'forename', 'born', 'surname', 'notes', 'died'}。


{
    'jgoodall':{'surname':'goodall',
                'forename':'Jane',
                'born':1934,
                'died':None,
                'notes':'primate researcher',
                'author':['In the Shadow of Man','The Chimpanzees of Gombe']},
    'rfranklin':{'surname':'Franklin',
                 'forename':'Rosalind',
                 'born':1920,
                 'died':1957,
                 'notes':'contributed to discovery of DNA'},
    'rcarson':{'surname':'Carson',
               'forename':'rachel',
               'born':1907,
               'died':1964,
               'notes':'raised awareness of effects of DDT',
               'author':['Silent Sporing']}
    }
''''''


太晚了明天写……