+ \begin{section}{Algebra}
+ If $R$ is a \index{commutative ring}, then $\polyring{R}{X,Y,Z}$
+ is a multivariate polynomial ring with indeterminates $X$, $Y$,
+ and $Z$, and coefficients in $R$. If $R$ is a moreover an integral
+ domain, then its fraction field is $\Frac{R}$.
+ \end{section}
+