added description of the lookup functions

book.tex

@@ -126,17 +126,19 @@ University.
 \chapter*{Preface}
 
 The tradition of compiler writing at Indiana University goes back to
-programming language research and courses taught by Daniel Friedman in
-the 1970's and 1980's. Dan had conducted research on lazy evaluation
-in the context of Lisp~\citep{McCarthy:1960dz} and then studied
-continuations and macros in the context of the
-Scheme~\citep{Sussman:1975ab}, a dialect of Lisp.  One of students of
-those courses, Kent Dybvig, went on to build Chez
+research and courses about programming languages by Daniel Friedman in
+the 1970's and 1980's. Dan had conducted research on lazy
+evaluation~\citep{Friedman:1976aa} in the context of
+Lisp~\citep{McCarthy:1960dz} and then studied
+continuations~\citep{Felleisen:kx} and
+macros~\citep{Kohlbecker:1986dk} in the context of the
+Scheme~\citep{Sussman:1975ab}, a dialect of Lisp.  One of the students
+of those courses, Kent Dybvig, went on to build Chez
 Scheme~\citep{Dybvig:2006aa}, a production-quality and efficient
 compiler for Scheme. After completing his Ph.D. at the University of
 North Carolina, Kent returned to teach at Indiana University.
-Throughout the 1990's and 2000's, Kent continued development of
-Chez Scheme and taught the compiler course.
+Throughout the 1990's and 2000's, Kent continued development of Chez
+Scheme and taught the compiler course.
 
 The compiler course evolved to incorporate novel pedagogical ideas
 while also including elements of effective real-world compilers.  One
@@ -144,7 +146,7 @@ of Dan's ideas was to split the compiler into many small ``passes'' so
 that the code for each pass would be easy to understood in isolation.
 (In contrast, most compilers of the time were organized into only a
 few monolithic passes for reasons of compile-time efficiency.)  Kent,
-with help later from his students Dipanwita Sarkar and Andrew Keep,
+with later help from his students Dipanwita Sarkar and Andrew Keep,
 developed infrastructure to support this approach and evolved the
 course, first to use micro-sized passes and then into even smaller
 nano passes~\citep{Sarkar:2004fk,Keep:2012aa}. Jeremy Siek took this
@@ -191,7 +193,7 @@ not had the chance to study in Bloomington in person.  Many of the
 compiler design decisions in this book are drawn from the assignment
 descriptions of \cite{Dybvig:2010aa}. We have captured what we think are
 the most important topics from \cite{Dybvig:2010aa} but we have omitted
-topics that I think are less interesting conceptually and we have made
+topics that we think are less interesting conceptually and we have made
 simplifications to reduce complexity.  In this way, this book leans
 more towards pedagogy than towards the absolute efficiency of the
 generated code. Also, the book differs in places where we saw the
@@ -265,8 +267,8 @@ language used to write the compiler.
 %
 Because this book uses Racket (\url{http://racket-lang.org}), a
 descendant of Lisp, we use S-expressions to represent programs
-(Section~\ref{sec:ast}), grammars to defined programming languages
-(Section~\ref{sec:grammar}), and pattern matching to inspect
+(Section~\ref{sec:ast}). We use grammars to defined programming languages
+(Section~\ref{sec:grammar}) and pattern matching to inspect
 individual nodes in an AST (Section~\ref{sec:pattern-matching}).  We
 use recursion to construct and deconstruct entire ASTs
 (Section~\ref{sec:recursion}).  This chapter provides an brief
@@ -6702,7 +6704,11 @@ Boolean \key{bool} is false.
 (define (assert msg bool) ...)
 \end{lstlisting}
 
-The \key{lookup} function ...
+The \key{lookup} function takes a key and an association list (a list
+of key-value pairs), and returns the first value that is associated
+with the given key, if there is one. If not, an error is triggered.
+The association list may contain both immutable pairs (built with
+\key{cons}) and mutable mapirs (built with \key{mcons}).
 
 The \key{map2} function ...