a	b	1
a	\N	1
c	d	1
c	\N	1
\N	\N	2
a	1
c	1
\N	2
a	b	1
a	\N	1
c	d	1
c	\N	1
\N	b	1
\N	d	1
\N	\N	2
a	1
c	1
\N	2