I try to understand how the tripartite Diffie-Hellman key exchange works. I read Joux's paper for this: https://www.semanticscholar.org/paper/A-One-Round-Protocol-for-Tripartite-Diffie-Hellman-Joux/845e96c20e5a5ff3b03f4caf72c3cb817a7fa542/pdf
However on page 388 he states the following equality which I don't see why it is true: $F_W(a,P_b,Q_c) = F_W(a,Q_b, P_c)$
where $F_W(x,y,z)= e_m(y,z)^x$. For me the first value of the equality is the inverse of the second one?