invocation of values for prolog execution does not require invocation of code_pred anymore

adding hotel keycard example for prolog generation

improving output of set comprehensions; adding style_check flags

improving ensure_groundness in prolog generation; added further example

adding very basic transformation to ensure groundness before negations

Markup_Tree.select: uniform treatment of root_range wrt. singularities, yielding empty result stream;

tuned signature;