minor edits

book.tex

@@ -130,10 +130,10 @@ Dedicated to PL group at Indiana University.
 The $S_0$ language includes integers, operations on integers,
 (arithmetic and input), and local variable definitions. This language
 is rich enough to exhibit several compilation techniques but simple
-enough so that we can reasonably hope to implement the compilation to
-x86-64 assembly in two weeks of hard work.  The instructor solution
+enough so that it is reasonable to implement the compilation to
+x86-64 assembly in two weeks of part-time work.  The instructor solution
 for the $S_0$ compiler consists of 6 recursive functions and a few
-small helper functions that together span 256 lines of code.
+helper functions that together span 256 lines of code.
 
 The syntax of the $S_0$ language is defined by the following grammar.
 \[
@@ -159,13 +159,17 @@ Given user input of 2, the following expression evaluates to $5$.
 (+ \; (\key{read}) \; 3)  \Longrightarrow 5
 \]
 Given user input of 2 then 3, the following expression may evaluate to
-either $1$ or $-1$ depending on the whims of the Scheme
-implementation.
+either $1$ or $-1$ depending on whether the implementation (an interpreter
+or compiler) decides to evaluate the \texttt{read} on the left before
+the \texttt{read} on the right, or vice versa.
 \[
 (+ \; (\key{read}) \; (-\; (\key{read}))) 
 \Longrightarrow
 1 \text{ or } -1
 \]
+We include the \texttt{read} operation in $S_0$ to demonstrate that
+order of evaluation sometimes makes a difference and also to prevent
+the use of an interpreter to trivially implement the compiler for $S_0$.
 
 \section{x86-64 Assembly}