\item[@"before"] A code fragment containing initialization to be performed at
the beginning of the effective method body. The default is to insert
nothing.
+\item[@"empty"] A code fragment executed if there are no primary methods;
+ it should usually store a suitable (identity) value in @<retvar>. The
+ default is not to emit an effective method at all if there are no primary
+ methods.
\item[@"first"] A code fragment to set the return value after calling the
first applicable direct method. The default is to use the @"each"
fragment.