Mention the co-authors of Helmut Heppe.
Who are the co-authors of Helmut Heppe?
SELECT DISTINCT ?answer WHERE { ?x <https://dblp.org/rdf/schema#authoredBy> <https://dblp.org/pid/253/0921> . ?x <https://dblp.org/rdf/schema#authoredBy> ?answer FILTER(?answer != <https://dblp.org/pid/253/0921>) }
<https://dblp.org/pid/253/0921>