code: protected final NodeIterator referringNodes( ReferenceType referenceType ) throws RepositoryException {
          if (!this.isReferenceable()) {
              return JcrEmptyNodeIterator.INSTANCE;
          }
  
          // Get all of the nodes that are referring to this node ...
          Set<NodeKey> keys = node().getReferrers(sessionCache(), referenceType);
          if (keys.isEmpty()) return JcrEmptyNodeIterator.INSTANCE;
          return new JcrNodeIterator(session(), keys.iterator(), keys.size(), null);
      }
docs: Obtain an iterator over the nodes that reference this node.
  
  @param referenceType specification of the type of references to include; may not be null
  @return the iterator over the referencing nodes; never null
  @throws RepositoryException if an error occurs while obtaining the information