code: public List<Relation> getRelations(Collection<Long> relationIds)
  	{
  		if(relationIds.isEmpty()) return Collections.emptyList();
  		return getSomeElements(RELATION + "s?" + RELATION + "s=" + toCommaList(relationIds), Relation.class);
  	}
docs: Note that if not logged in, the Changeset for each returned element will be null
  
  @param relationIds a collection of relation ids to return
  @throws OsmNotFoundException if <b>any</b> one of the given relations does not exist
  @return a list of relations.