s,p,o,p2,o2
http://example.org/s1,http://example.org/p1,http://example.org/s2,http://example.org/p2,foo
http://example.org/s2,http://example.org/p2,foo,,
http://example.org/s3,http://example.org/p3,bar,,
http://example.org/s4,http://example.org/p4,4,,
http://example.org/s5,http://example.org/p5,5.5,,
http://example.org/s6,http://example.org/p6,_:a,,