rPref 0.3
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* Translation of preferences to other preference query languages via `show.query`

* Optimized evaluation of preferences with long prioritization chains

* Added neutral element `empty()` for preference composition

* Fixed memory leak in preference evaulation

* Fixed bug: wrong Hasse diagramms for some kinds of preferences in get_btg

* Fixed bug: `psel` failed on empty dataframes

* Hasse diagramm can be returned as adjacency list via `get_hasse_diag`