On Mac OS X, underscores are required starting from instruction selection because the emulator fails without them

book.tex

@@ -3561,6 +3561,13 @@ For $\SEQ{s}{t}$, you can translate the statement $s$ and tail $t$
 recursively and then append the resulting instructions.
 \fi}
 
+{\if\edition\pythonEd
+We recommend that you use the function \code{utils.label\_name()} to
+transform a string into an label argument suitably suitable for, e.g.,
+the target of the \code{callq} instruction. This practice makes your
+compiler portable across Linus and Mac OS X, which requires an underscore prefixed to
+all labels.
+\fi}
 \begin{exercise}
   \normalfont
 {\if\edition\racketEd