I realize that this is very trivial, but I can't seem to figure out the notation used. I'm sorry for asking such a trivial question. Here's a link ot the book for reference http://csee.wvu.edu/~zheng/GraphTheoryIII.pdf
I have been reading the Diestel Graph Theory (3rd edition) book and came across this proof of Menger's theorem
The symbol I don't understand is the one with the arrow pointing to it. I looked at the symbol index, but that was no help. I was hoping that this was commonly used notation, or that someone might be able to deduce what it means. I would really appreciate the help. Thank You.