[Haskell-cafe] Problem over Context Free Grammar Transitive Closure Algorithm

Folks this is a parser for Context Free Grammars, for some reason when I go simplify the grammar trying to remove productions that substitute variables like in the case: S -> A, A -> B, B -> a, it works perfectly and returns S -> a, the problem is when the root symbol is on the listthat must be replaced as in: S -> aXa | bXb , X -> a | b | S | £(empty) instead of returning S -> aXa | bXb, X -> a | b | £ | aXa | bXb it ruturns X as X -> a | b | £ | S <- this S is wrong! it should return X -> a | b | £ | aXa | bXb! Im not sure about where is the mistake. If you can give me hints it would be grat. I will be checking the mail and also hanging over #haskell at freenode ifyou want to contact me directly. Thanks in advance for the help! Victor