code: public SmiOidNode getRootNode() {
          SmiOidNode parent = this;
          while (parent.getParent() != null) {
              parent = parent.getParent();
          }
          return parent;
      }
docs: This method as such makes no sense (you can get the root node directly from SmiMib),
  but it is used mainly for unit testing.
  
  @return The root of the oid tree.