[antlr-interest] ANTLR 3.1: Help: Why do I get this warning

[Francis ANDRE]

Hi ANTLRs

While I am not using any rewriting rule or operator, ANTLR emits a warning
ANTLR Parser Generator  Version 3.1 (August 12, 2008)  1989-2008
warning(149): Cobol.g:0:0: rewrite syntax or operator with no output 
option; setting output=AST

Why do I get such warning??

grammar Cobol;
options {
    k=4;
}

tokens {
    /*A*/
    ACCEPT                ='ACCEPT';
...
....