code: @Nullable
    private static Node getOnlyInterestingChild(Node block) {
      if (!block.isBlock()) {
        return null;
      }
      if (block.hasOneChild()) {
        return block.getOnlyChild();
      }
  
      Node ret = null;
      for (Node child : block.children()) {
        if (child.isSyntheticBlock() && !child.hasChildren()) {
          // Uninteresting child.
        } else if (ret != null) {
          // Found more than one interesting child.
          return null;
        } else {
          ret = child;
        }
      }
      return ret;
    }
docs: Return the only "interesting" child of {@code block}, if it has exactly one interesting child,
  otherwise return null. For purposes of this method, a node is considered "interesting" unless
  it is an empty synthetic block.