Bug in Gosper (correction)? (was: Symbolic summation)

>
> Example by: Miguel
>
> load(zeilberger);
>
> a(n):=(n^3+2*n+7*n+5)/(n^8+8*n^5+6*n^4+2*n^3+n^2+n+11);
> b(n):=a(n)-a(n+1);
>
> b(n);
>
> AntiDifference(%);
>
> A strange error message follows: "Unknown operator : -"!
>
>   

Sorry, the bug seems to be triggered by

load(zeilberger);
a(n):=1/n;
b(n):=a(n+1)-a(n);
b(n);
AntiDifference(%,n);

In contrast

load(zeilberger);
a(n):=1/n;
b(n):=a(n)-a(n+1);
b(n);
AntiDifference(%,n);

works fine.

Michel