+There are three kinds of layout information:
+
+\begin{itemize}
+\item Standard layout information is for closures which place pointers
+before non-pointers in instances of the closure (this applies to most
+heap-based and static closures, but not activation records). The
+layout information for standard closures is
+
+ \begin{itemize}
+ \item Number of pointer fields (16 bits).
+ \item Number of non-pointer fields (16 bits).
+ \end{itemize}
+
+\item Activation records don't have pointers before non-pointers,
+since stack-stubbing requires that the record has holes in it. The
+layout is therefore represented by a bitmap in which each '1' bit
+represents a non-pointer word. This kind of layout info is used for
+@RET_SMALL@ and @RET_VEC_SMALL@ closures.
+
+\item If an activation record is longer than 32 words, then the layout
+field contains a pointer to a bitmap record, consisting of a length
+field followed by two or more bitmap words. This layout information
+is used for @RET_BIG@ and @RET_VEC_BIG@ closures.
+
+\item Selector Thunks (\secref{THUNK_SEL}) use the closure
+layout field to hold the selector index, since the layout is always
+known (the closure contains a single pointer field).
+\end{itemize}