\begin{xy} \xymatrix{ & x \ar[ld]_{ {\rm coeval}_{x,y}} \ar[rd]^{ {\rm coeval}_{x,y'}} \\ (x \otimes y)^y \ar[rd]_{(x \otimes g)^y} & & (x \otimes y')^{y'} \ar[ld]^{(x \otimes y')^g} \\ & (x \otimes y')^y } \end{xy}