typo

book.tex

@@ -2904,7 +2904,7 @@ user-defined names.
 \begin{array}{rcl}
 \Atm &::=& \INT{\Int} \mid \VAR{\Var} \\
 \Exp &::=& \Atm \mid \READ{} \\
-n     &\mid& \NEG{\Atm} \mid \ADD{\Atm}{\Atm}  \\
+     &\mid& \NEG{\Atm} \mid \ADD{\Atm}{\Atm}  \\
      &\mid&  \LET{\Var}{\Exp}{\Exp} \\
 R^{\dagger}_1  &::=& \PROGRAM{\code{'()}}{\Exp}
 \end{array}