Added warning about opening scsh structure before scheme structure

in modules.

doc/scsh-manual/running.tex

@@ -145,6 +145,42 @@ Although scsh can be used without referring to the module system at
 all, we recommend taking the time to learn and use it.
 The effort will pay off in the construction of modular, factorable programs.
 
+\subsubsection{Module warning}
+Programmers who open both the \ex{scheme} and \ex{scsh} structures in their
+own packages should make sure to always put the \ex{scsh} reference first.
+\begin{center}
+\begin{tabular}{l@{\qquad}l}
+Do this: & Not this: \strut \\
+\quad{\begin{codebox}[b]
+(define-structure web-server
+  (open scsh
+        scheme
+        net-hax
+        \vdots)
+  (file web))\end{codebox}}
+&
+\quad{\begin{codebox}[b]
+(define-structure web-server
+  (open scheme
+        scsh
+        net-hax
+        \vdots)
+  (file web))\end{codebox}}\\
+%
+Open \ex{scsh} before \ex{scheme}. &
+Not \ex{scsh} after \ex{scheme}.
+\end{tabular}
+\end{center}
+Ordering the two packages like this is necessary because scsh overrides
+some of the standard R4RS Scheme definitions exported by the \ex{scheme}
+package with its own definitions.
+For example, scsh's versions of the R4RS I/O functions such as \ex{display}
+and \ex{write} take integer file descriptors as arguments, as well as Scheme
+ports.
+If you open the \ex{scheme} structure before the \ex{scsh} structure,
+you'll get the standard {\scm} definitions, which is not what you want.
+
+
 \subsection{Switches}
 \label{sec:scsh-switches}
 The scsh top-level takes command-line switches in the following format: