(VAR F G H T X )
(RULES 
         -> 
        implode(nil, F, X) -> X
        implode(cons(H, T), F, X) -> implode(T, F, )
        explode(nil, F, X) -> X
        explode(cons(H, T), F, X) -> explode(T, op(F, F), )
        
)