code: public AbstractJcrNode getNonSystemNodeByIdentifier( String id ) throws ItemNotFoundException, RepositoryException {
          checkLive();
          if (NodeKey.isValidFormat(id)) {
              // Try the identifier as a node key ...
              try {
                  NodeKey key = new NodeKey(id);
                  return node(key, null);
              } catch (ItemNotFoundException e) {
                  // continue ...
              }
          }
          // Try as node key identifier ...
          NodeKey key = this.rootNode.key.withId(id);
          return node(key, null);
      }
docs: A variant of the standard {@link #getNodeByIdentifier(String)} method that does <i>not</i> find nodes within the system
  area. This is often needed by the {@link JcrVersionManager} functionality.
  
  @param id the string identifier
  @return the node; never null
  @throws ItemNotFoundException if a node cannot be found in the non-system content of the repository
  @throws RepositoryException if there is another problem
  @see #getNodeByIdentifier(String)