[Maxima] Bug in Gosper? (was: Symbolic summation)

[Michel Van den Bergh]

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 : -"!

Michel