code: private RootSegmentBase validate() {
  
          ContainerSegmentBase root = containerStack.peekFirst();
  
          if (!(root instanceof RootSegmentBase)) {
              throw new MustacheException(
                      MustacheProblem.COMPILE_INVALID_TEMPLATE,
                      "Incorrect last container segment on the stack: %s",
                      containerStack.peekFirst().toString(), line);
          }
          return (RootSegmentBase) root;
      }
docs: Validate the compiled template.