>>> from clotheshop import stock_decrease

>>> d = {('Pan85', 'XS','black'):100,('Pan85','S','black'):300,
... ('Pan85','XS','blue'):6,('Pan87','S','blue'):5,
... ('Pan87','L','blue'):12}
>>> stock_decrease(d,'Pan85','XS','blue',2)
>>> if d != {('Pan85', 'XS','black'):100,('Pan85','S','black'):300,
... ('Pan85','XS','blue'):4,('Pan87','S','blue'):5,
... ('Pan87','L','blue'):12}:
...    print(d)