diff --git a/Doc/lib/libsys.tex b/Doc/lib/libsys.tex
index 1e256af..46c0186 100644
--- a/Doc/lib/libsys.tex
+++ b/Doc/lib/libsys.tex
@@ -167,7 +167,10 @@
   Strings specifying the primary and secondary prompt of the
   interpreter.  These are only defined if the interpreter is in
   interactive mode.  Their initial values in this case are
-  \code{'>>> '} and \code{'... '}.
+  \code{'>>> '} and \code{'... '}.  If a non-string object is assigned
+  to either variable, its \code{str()} is re-evaluated each time the
+  interpreter prepares to read a new interactive command; this can be
+  used to implement a dynamic prompt.
 \end{datadesc}
 
 \begin{funcdesc}{setcheckinterval}{interval}