Documented class exceptions.
diff --git a/Doc/ref/ref4.tex b/Doc/ref/ref4.tex
index ed621c4..4b4d522 100644
--- a/Doc/ref/ref4.tex
+++ b/Doc/ref/ref4.tex
@@ -169,13 +169,33 @@
When an exception is not handled at all, the interpreter terminates
execution of the program, or returns to its interactive main loop.
-Exceptions are identified by string objects. Two different string
-objects with the same value identify different exceptions.
+Exceptions are identified by string objects or class instances. Two
+different string objects with the same value identify different
+exceptions. An exception can be raised with a class instance. Such
+exceptions are caught by specifying an except clause that has the
+class name (or a base class) as the condition.
When an exception is raised, an object (maybe \verb@None@) is passed
as the exception's ``parameter''; this object does not affect the
selection of an exception handler, but is passed to the selected
-exception handler as additional information.
+exception handler as additional information. For exceptions raised
+with a class instance, the instance is passed as the ``parameter''.
+
+For example:
+
+\begin{verbatim}
+>>> class Error:
+... def __init__(self, msg): self.msg = msg
+...
+>>> class SpecificError(Error): pass
+...
+>>> try:
+... raise SpecificError('broken')
+... except Error, obj:
+... print obj.msg
+...
+broken
+\end{verbatim}
See also the description of the \verb@try@ and \verb@raise@
statements.