fix partial eval, issue #215

Author: jsiek

book.tex

@@ -23,7 +23,7 @@
 
 \def\racketEd{0}
 \def\pythonEd{1}
-\def\edition{0}
+\def\edition{1}
 
 % material that is specific to the Racket edition of the book
 \newcommand{\racket}[1]{{\if\edition\racketEd{#1}\fi}}
@@ -1723,7 +1723,7 @@ \section{Example Compiler: A Partial Evaluator}
     case BinOp(left, Add(), right):
       return pe_add(pe_exp(left), pe_exp(right))
     case BinOp(left, Sub(), right):
-      return pe_add(pe_exp(left), pe_neg(right))
+      return pe_add(pe_exp(left), pe_neg(pe_exp(right)))
     case UnaryOp(USub(), v):
       return pe_neg(pe_exp(v))
     case Constant(value):