clarify program node

book.tex

@@ -1575,8 +1575,12 @@ test your passes on the example programs.
 In the \key{select-instructions} pass we begin the work of translating
 from $C_0$ to x86. The target language of this pass is a pseudo-x86
 language that still uses variables, so we add an AST node of the form
-$\VAR{\itm{var}}$ to the x86 abstract syntax.  The
-\key{select-instructions} pass deals with the differing format of
+$\VAR{\itm{var}}$ to the x86 abstract syntax. Also, the \key{program}
+form should still list the variables (similar to $C_0$):
+\[
+  (\key{program}\;(\Var^{*})\;\Instr^{+})
+\]
+The \key{select-instructions} pass deals with the differing format of
 arithmetic operations. For example, in $C_0$ an addition operation can
 take the form below.  To translate to x86, we need to use the
 \key{addq} instruction which does an in-place update. So we must first
@@ -1687,11 +1691,15 @@ above to
 \end{lstlisting}
 
 In the process of assigning stack locations to variables, it is
-convenient to compute and store the size of the frame in the
-$\itm{info}$ field of the \key{program} node which will be needed
-later to generate the procedure conclusion. Some operating systems
-place restrictions on the frame size. For example, Mac OS X requires
-the frame size to be a multiple of 16 bytes.
+convenient to compute and store the size of the frame in the first
+field of the \key{program} node which will be needed later to generate
+the procedure conclusion. 
+\[
+  (\key{program}\;\Int\;\Instr^{+})
+\]
+Some operating systems place restrictions on
+the frame size. For example, Mac OS X requires the frame size to be a
+multiple of 16 bytes.
 
 \begin{exercise}
 \normalfont Implement the \key{assign-homes} pass and test it on all