code: public IndexedSet<I> involvedItems() {
  		IndexedSet<I> res = allItems.empty();
  		res.indices().addAll(matrix.involvedCols());
  		return res;
  	}
docs: Gets the set of items in {@link #allItems()} that are contained in at
  least one transaction
  
  @return the set of items in {@link #allItems()} that are contained in at
  least one transaction