>>> from students import listed

>>> dic = {('Pol','Amber','Coll'):['PHY','PE','CHM'],
... ('Aina','Soler','Gil'):['PHY','PE','CHM','MTH'],
... ('Joan','Rubio','Boix'):['PE','CHM','MTH', 'ENG'],
... ('Mar','Pou','Coll'):['PE','GE']}
>>> listed(dic, 'PHY')
['Amber Coll, Pol', 'Soler Gil, Aina']
>>> listed(dic, 'PR')
[]