>>> from botigaroba import decrementa_estoc

>>> d = {('Pan85', 'XS','negre'):100,('Pan85','S','negre'):300,
... ('Pan85','XS','blau'):6,('Pan87','S','blau'):5,
... ('Pan87','L','blau'):12}
>>> decrementa_estoc(d,'Pan85','XS','blau',2)
>>> d == {('Pan85', 'XS','negre'):100,('Pan85','S','negre'):300,
... ('Pan85','XS','blau'):4,('Pan87','S','blau'):5,
... ('Pan87','L','blau'):12}
True