For this "mostly used for debugging" filter, it probably isn't worth much of a debate, but ... say JSON and I think that many people (including some of my students) immediately think of a string representation. Which, IMHO, is natural because the named JSON is for the "notation" (syntax).

Anyhow, I was thinking about this after I submitted the PR. **A better fix would be to rename this filter to `toJson`** (this would match naming conventions used elsewhere) ... but since I cannot run tests (and since this would be a "breaking change") I decided to leave the name as is for now.