gutters in the left margin

Hi,

Sometime in the past year or so my .R files have gained a small gutter/margin on the left margin. At first I thought it was something in ESS, but when I tried without my config it's not there. But it only shows up in .R buffers, so I'm guessing it's something in the ESS/polymode/etc universe.

I've attached a screenshot. I'm not sure the mailing list supports attachments, so you can also see it here:

https://flic.kr/p/2qALDtW

Does anybody know where this feature comes from - which package or setting is involved?

Thanks for your patience with my very vague and probably Off-topic query!

- tyler

-- 
plantarum.ca

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