/**
  * Returns the set of incomplete members of the given relations.
  *
  * @param rels The relations to inspect.
  * @return The set of incomplete members of the given relations.
  */
 public Set<OsmPrimitive> buildSetOfIncompleteMembers(Collection<Relation> rels) {
   Set<OsmPrimitive> ret = new HashSet<OsmPrimitive>();
   for (Relation r : rels) {
     ret.addAll(r.getIncompleteMembers());
   }
   return ret;
 }