code: private void appendDirectives(StringBuffer query) {
  		StringBuffer sb = new StringBuffer();
  		for (String prefix : prefixes.keySet()) {
  			sb.append("@PREFIX");
  			sb.append(" ");
  			sb.append(prefix);
  			sb.append(" ");
  			sb.append("<");
  			sb.append(prefixes.get(prefix));
  			sb.append(">");
  			sb.append(" .\n");
  		}
  		sb.append("@PREFIX " + OntopInternal.PREFIX_XSD + " <" + XSD.PREFIX + "> .\n");
  		sb.append("@PREFIX " + OntopInternal.PREFIX_OBDA + " <" + Ontop.PREFIX + "> .\n");
  		sb.append("@PREFIX " + OntopInternal.PREFIX_RDF + " <" + RDF.PREFIX + "> .\n");
  		sb.append("@PREFIX " + OntopInternal.PREFIX_RDFS + " <" + RDFS.PREFIX + "> .\n");
  		sb.append("@PREFIX " + OntopInternal.PREFIX_OWL + " <" + OWL.PREFIX + "> .\n");
  		query.insert(0, sb);
  	}
docs: The turtle syntax predefines the quest, rdf, rdfs and owl prefixes.
  
  Adds directives to the query header from the PrefixManager.