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ToEmanuel Sperner on his 65th birthday
This work has been partially supported by National Research Council of Canada grant no. A 7251.
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Ellers, E., Row, D. Hyperfree extensions. Abh.Math.Semin.Univ.Hambg. 37, 46–49 (1972). https://doi.org/10.1007/BF02993898
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DOI: https://doi.org/10.1007/BF02993898