code: PolicyNode getPolicyTree() {
          if (rootNode == null)
              return null;
          else {
              PolicyNodeImpl policyTree = rootNode.copyTree();
              policyTree.setImmutable();
              return policyTree;
          }
      }
docs: Gets the root node of the valid policy tree, or null if the
  valid policy tree is null. Marks each node of the returned tree
  immutable and thread-safe.
  
  @returns the root node of the valid policy tree, or null if
  the valid policy tree is null