a	('b','c')	('b','c')
d	('e','f')	('','')
a
x
a	('b','c')	('b','c')
x	('','')	('y','z')
a
d
a
x
a	('b','c')	('b','c')
d	('e','f')	('','')
a	('b','c')	('b','c')
x	('','')	('y','z')
a	b	['b','c']
d	e	[]
a	b	['b','c']
x		['y','z']
a
d
a
x
a	b	['b','c']
d	e	[]
a	b	['b','c']
x	\N	['y','z']