In 2013 in PPDP, what are the titles of the papers on Type extensions?
What are the titles of the papers on Type extensions that were published in International Conference on Principles and Practice of Declarative Programming in 2013?
SELECT DISTINCT ?answer WHERE { <https://dblp.org/rec/conf/ppdp/CaballeroST13> <https://dblp.org/rdf/schema#publishedIn> 'PPDP' . <https://dblp.org/rec/conf/ppdp/CaballeroST13> <https://dblp.org/rdf/schema#yearOfPublication> '2013' . <https://dblp.org/rec/conf/ppdp/CaballeroST13> <https://dblp.org/rdf/schema#title> ?answer }
<https://dblp.org/rec/conf/ppdp/CaballeroST13>