\begin{description}
\item[@|message_class|] A symbol naming the Lisp class to use to represent
the message.
+\item[@|readonly|] A boolean indicating whether the message guarantees not to
+ modify its receiver. If this is true, the receiver will be declared
+ @"const".
\item[@|combination|] A keyword naming the aggregating method combination to
use.
\item[@|most_specific|] A keyword, either @`first' or @`last', according to