Add index entry for 'cast'.

doc/cln.tex

@@ -775,6 +775,7 @@ Conversions from any class to any of its subclasses (``derived classes'' in
 C++ terminology) are not provided. Instead, you can assert and check
 that a value belongs to a certain subclass, and return it as element of that
 class, using the @samp{As} and @samp{The} macros.
+@cindex cast
 @cindex @code{As()()}
 @code{As(@var{type})(@var{value})} checks that @var{value} belongs to
 @var{type} and returns it as such.