usage of CLASS snacc

hello friends,

I'd like to use the CLASS keyword (to define operations) and snacc does not
like the normal syntax 
   OPERATION ::= CLASS
                 {
                  &ArgumentType          OPTIONAL,
                  &ResultType            OPTIONAL,
                  &Errors                ERROR OPTIONAL,
                  &LinkedOperations      OPTIONAL,
                  &operationCode         Code UNIQUE
                 }

does anybody know why? and what should be done?

Bizhan