Question about proof nets

Hi, everyone. I am currently trying to understand (tensor, cotensor)-circuits from R. F. Blute, J. Cockett, R. Seely, and T. H. Trimble. Natural deduction and coherence for weakly distributive categories. Journal of Pure and Applied Algebra, 113(3):229–296, 1996. I am wondering if anyone can tell me what the significance of the half circle under the tensor in the circuit for tensor elimination? Unfortunately, I cannot draw the circuit here. It is however on page 9 of the above paper. At first I thought it was indicating symmetry, but this doesn't make sense. Any help would be greatly appreciated. .\ Harley [For admin and other information see: http://www.mta.ca/~cat-dist/ ]