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[Hem98] Harald Hempel. Boolean Hierarchies – On Collapse Properties and Query Order. PhD thesis, Friedrich-Schiller Universität Jena, Germany, 1998. 
@phdthesis{phd-hempel,
  author =              {Hempel, Harald},
  title =               {Boolean Hierarchies~-- On~Collapse Properties and
                         Query Order},
  year =                {1998},
  school =              {Friedrich-Schiller Universit{\"a}t Jena, Germany},
}
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