code: public static SplittableElementSet<Node> newNodeIndex(Collection<Node> c, TIntIntHashMap idx) {
          return new SplittableElementSet<>(c, idx);
      }
docs: Make a new splittable set from a collection of nodes.
  We consider the collection does not have duplicated elements.
  
  @param c   the collection to wrap
  @param idx the partition for each node
  @return the resulting set