Thank you for proposing a contribution to the ANTLR project. In order to accept changes from the outside world, all contributors most "sign" the  [contributors.txt](https://github.com/antlr/antlr4/blob/master/contributors.txt) contributors certificate of origin. It's an unfortunate reality of today's fuzzy and bizarre world of open-source ownership.

Make sure you are already in the contributors.txt file or add a commit to this pull request with the appropriate change. Thanks!