code: public final Node useSourceInfoIfMissingFromForTree(Node other) {
      useSourceInfoIfMissingFrom(other);
      for (Node child = first; child != null; child = child.next) {
        child.useSourceInfoIfMissingFromForTree(other);
      }
  
      return this;
    }
docs: Overwrite all the source information in this node and its subtree with
  that of {@code other} iff the source info is missing.