rPref 0.5
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* New functions for string output (pref.str) and partial evaluation (eval.pref)

* Predecessor/successor (hasse_pred, etc.) result vectors are ordered ascending

* New functionality of predecessor/successor functions (empty vectors, new parameter "intersect")

* New base preferences for defining preferences on expressions/symbols (low_, high_, true_)

* Fixed bug in multithreaded calculation of preferences