This isn't your doing, but I wonder why the Mozilla source map library [defaults to the version being 3](https://github.com/mozilla/source-map/blob/master/lib/source-map/source-map-generator.js#L36), and suggests that it's private by prefixing it with an underscore. The way I interpret [the specs](https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit?hl=en_US&pli=1&pli=1) is that this should increment with each successive source map. I guess they defaulted to this because the spec uses the example of 3, and it happens to be the 3rd revision of the spec. Or maybe I'm just misinterpreting it. What do you think, @mzgoddard? I can raise an issue on the repo if you think there's anything to this thought.