[R] Clarification on debug output

[Mick Jordan]

I hope someone can explain what the #1 means (and for that matter the 
[2] in the debug output below. I can't find anything in the spec that 
explains what they mean.

Thanks

 > f(1)
debugging in: f(1)
debugging in: f()
debug at #1: {
     if (x < 0)
         -x
     else x
}
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