meaning of + sign followed by prompt

[Sebastian Luque]

Dear list members,

I'm using ESS+Emacs and don't understand why I'm getting this message. 
The '+' sign followed by the prompt '>' is displayed after sending a 
line from the editor to R. I'm getting this message when sending one 
particular command to R, assigning a function to an object. The odd 
thing is that everything seems to be fine except for this message; the 
object correctly assigned the result of the function to the object. I'm 
working with dozens of very similar assignments (they have the same 
structure), but it's only with this particular one that the message 
appears. I could just move on, since it's not affecting the final 
product, but this may be a sign of some inconsistency. I'd greatly 
appreciate any clues as to what the problem might be (although there's 
none superficially!).

Thanks in advance,
Sebastian