fix description of S

book.tex

@@ -26,7 +26,7 @@
 
 \def\racketEd{0}
 \def\pythonEd{1}
-\def\edition{1}
+\def\edition{0}
 
 % material that is specific to the Racket edition of the book
 \newcommand{\racket}[1]{{\if\edition\racketEd{#1}\fi}}
@@ -6712,9 +6712,12 @@ make sure to take into account the space used for saving the
 callee-saved registers. Also, remember that the frame needs to be a
 multiple of 16 bytes! We recommend using the following equation for
 the amount $A$ to subtract from the \code{rsp}. Let $S$ be the number
-of spilled variables and $C$ be the number of callee-saved registers
-that were allocated to variables. The $\itm{align}$ function rounds a
-number up to the nearest 16 bytes.
+of stack locations used by spilled variables\footnote{Sometimes two or
+  more spilled variables are assigned to the same stack location, so
+  $S$ can be less than the number of spilled variables.} and $C$ be
+the number of callee-saved registers that were allocated to
+variables. The $\itm{align}$ function rounds a number up to the
+nearest 16 bytes.
 \[
    \itm{A} = \itm{align}(8\itm{S} + 8\itm{C}) - 8\itm{C}
 \]