[ESS] Arrow in left margin

[Rich Shepard]

   Running Slackware-14.1 with emacs-24.3 and ESS-13.09. What just showed up
a couple of days ago is an arrow (short horizontal line with vertical arrow
on the left end) in the left margin, before the R prompt. See the attached
screen shot. It's not there on each line, but seems to come and go.

   I'm curious what it signifies. If anyone knows, please share your
knowledge with me.

TIA,

Rich
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