address a comment from Jeremy re Rvar's overflow semantics

book.tex

@@ -1414,7 +1414,9 @@ exit with a return code of \code{255}.  The interpreters in chapters
 \begin{ocamlx}
   In OCaml, overflow does not cause a trap; instead values ``wrap around''
   to produce results modulo $2^{63}$.  The result of this program is
-  \key{-1223372036854775816}.
+  \key{-1223372036854775816}.  We will embrace this wrap-around behavior 
+  as the intended one for \LangInt{}, so the OCaml version will have
+  no undefined behaviors due to overflow. 
 \end{ocamlx}
 
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