R1: - is an unary op

book.tex

@@ -1083,8 +1083,8 @@ exhibit several compilation techniques.
 \[
 \begin{array}{rcl}
   \Exp &::=& \Int \mid (\key{read}) \mid (\key{-}\;\Exp) \mid (\key{+} \; \Exp\;\Exp)
-        \mid (\key{-}\;\Exp\;\Exp) \\
-     &\mid&  \Var \mid (\key{let}~([\Var~\Exp])~\Exp) \\
+        \mid \Var \\
+     &\mid& (\key{let}~([\Var~\Exp])~\Exp) \\
   R_1 &::=& \Exp
 \end{array}
 \]