@article{10_4064_ba57_3_2,
     author = {Arthur W. Apter},
     title = {$L$-like {Combinatorial} {Principles}  and {Level} by {Level} {Equivalence}},
     journal = {Bulletin of the Polish Academy of Sciences. Mathematics},
     pages = {199--207},
     publisher = {mathdoc},
     volume = {57},
     number = {3},
     year = {2009},
     doi = {10.4064/ba57-3-2},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ba57-3-2/}
}