code: public static <T> Paging<T> flattenPaging(Collection<Paging<T>> collectionOfPaging,
  	Comparator<T> sorter, BooleanExpression<T> filter)
  	{
  		if (collectionOfPaging == null || collectionOfPaging.isEmpty())
  			return null;
  
  		PageCriteria pageCriteria = collectionOfPaging.iterator().next().getPageCriteria();
  		return flattenPaging(collectionOfPaging, pageCriteria, sorter, filter);
  	}
docs: Aggregates multiple collections into a single paging collection
  
  @param collectionOfPaging
  the collection paging that need to be flatten
  @param sorter
  optional comparable for sorting
  @param filter
  optional filter, if filter.
  @param <T>
  the type class
  @return the flatten collections into a single collection