(RULES
a  -> b ,
a a  -> a b a ,
a b  -> b b b ,
a a a  -> a a b a a ,
a a b  -> a b b a b ,
a b a  -> b a b b a ,
a b b  -> b b b b b ,
a a a a  -> a a a b a a a ,
a a a b  -> a a b b a a b ,
a a b a  -> a b a b a b a ,
a a b b  -> a b b b a b b ,
a b a a  -> b a a b b a a ,
a b a b  -> b a b b b a b ,
a b b a  -> b b a b b b a ,
a b b b  -> b b b b b b b )
(COMMENT: extended SRS aX -> XbX for all
strings X of length at most 3)