more specific about \Arg

book.tex

@@ -1504,25 +1504,26 @@ $\Rightarrow$
 We recommend implementing \key{flatten} as a structurally recursive
 function that returns two things, 1) the newly flattened expression,
 and 2) a list of assignment statements, one for each of the new
-variables introduced while flattening the expression.  The newly
-flattened expression should be a \emph{simple} expression, that is, an
-integer or a variable. (There will be more kinds of simple expressions
-in the input languages of later Chapters.) You can return multiple
-things from a function using the \key{values} form and you can receive
-multiple things from a function call using the \key{define-values}
-form. If you are not familiar with these constructs, the Racket
-documentation will be of help. Also, the \key{map2} function
-(Appendix~\ref{appendix:utilities}) is useful for applying a function
-to each element of a list, in the case where the function returns two
-values. The result of \key{map2} is two lists.
+variables introduced during the flattening the expression.  The newly
+flattened expression should be an $\Arg$ in the $C_0$ syntax
+(Figure~\ref{fig:c0-syntax}), that is, it should be an integer or a
+variable. You can return multiple things from a function using the
+\key{values} form and you can receive multiple things from a function
+call using the \key{define-values} form. If you are not familiar with
+these constructs, the Racket documentation will be of help. Also, the
+\key{map2} function (Appendix~\ref{appendix:utilities}) is useful for
+applying a function to each element of a list, in the case where the
+function returns two values. The result of \key{map2} is two lists.
 
 The clause of \key{flatten} for the \key{program} node needs to
-recursively flatten the body of the program and also compute the list
-of variables used in the program.  I recommend traversing the
-statements in the body of the program (after it has been flattened)
-and collect all variables that appear on the left-hand-side of an
-assignment. Note that each variable should only occur once in the list
-of variables that you place in the \key{program} form.
+recursively flatten the body of the program and the newly flattened
+expression should be placed in a \key{return} statment.  The
+\key{flatten} pass should also compute the list of variables used in
+the program.  I recommend traversing the statements in the body of the
+program (after it has been flattened) and collect all variables that
+appear on the left-hand-side of an assignment. Note that each variable
+should only occur once in the list of variables that you place in the
+\key{program} form.
 
 Take special care for programs such as the following that initialize
 variables with integers or other variables. It should be translated
@@ -2652,17 +2653,18 @@ $\Rightarrow$
 \end{tabular} \\
 The value of the \key{if} expression is the value of the branch that
 is selected. Recall that in the \code{flatten} pass we need to replace
-complex expressions with simple expressions (variables or
-literals). In the translation above, on the right, we have translated
-the \key{if} expression into a new variable \key{if.1} and we have
-produced code that will assign the appropriate value to \key{if.1}.
-For $R_1$, the \code{flatten} pass returned a list of assignment
-statements. Here, for $R_2$, we return a list of statements that can
-include both \key{if} statements and assignment statements.
+arbitrary expressions with $\Arg$'s (variables or literals). In the
+translation above, on the right, we have translated the \key{if}
+expression into a new variable \key{if.1} and we have produced code
+that will assign the appropriate value to \key{if.1}.  For $R_1$, the
+\code{flatten} pass returned a list of assignment statements. Here,
+for $R_2$, we return a list of statements that can include both
+\key{if} statements and assignment statements.
 
 The next example is a bit more involved, showing what happens when
-there are complex expressions in the condition and branch expressions
-of an \key{if}, including nested \key{if} expressions.
+there are complex expressions (not variables or literals) in the
+condition and branch expressions of an \key{if}, including nested
+\key{if} expressions.
 
 \begin{tabular}{lll}
 \begin{minipage}{0.4\textwidth}