fix to the inert grammar, the case for let

book.tex

@@ -2712,7 +2712,7 @@ subexpression is a constant.
 \itm{inert} &::=& \Var \mid \LP\key{read}\RP \mid \LP\key{-} \;\Var\RP
     \mid \LP\key{-} \;\LP\key{read}\RP\RP
     \mid \LP\key{+} \; \itm{inert} \; \itm{inert}\RP\\
-  &\mid& \LP\key{let}~\LP\LS\Var~\itm{inert}\RS\RP~ \itm{inert} \RP \\
+  &\mid& \LP\key{let}~\LP\LS\Var~\itm{inert}\RS\RP~ \itm{residual} \RP \\
 \itm{residual} &::=& \Int \mid \LP\key{+}\; \Int\; \itm{inert}\RP \mid \itm{inert} 
 \end{array}
 \]