Clarify Cartesian-product operator

frap_book.tex

@@ -217,6 +217,8 @@ Such types can be defined by enumerating their \emph{constructor}\index{construc
   \mathsf{Times} &:& \mathsf{Exp} \times \mathsf{Exp} \to \mathsf{Exp}
 \end{eqnarray*}
 
+Note that the ``$\times$'' here is not the multiplication operator of concrete syntax, but rather the Cartesian-product operator\index{Cartesian product} of set theory, to indicate a type of pairs!
+
 Such a list of constructors defines the set $\mathsf{Exp}$ to contain exactly those terms that can be built up with the constructors.
 In inference-rule notation:
 \encoding