Maxima Function
Zeilberger (F_{n,k}, k, n)
Attempts to compute the indefinite hypergeometric summation of F_{n,k}.
Zeilberger first invokes Gosper, and if that fails to find a solution, then invokes
parGosper with order 1, 2, 3, ..., up to MAX_ORD.
If Zeilberger finds a solution before reaching MAX_ORD,
it stops and returns the solution.
The algorithms yields a sequence [s_1, s_2, ..., s_m] of solutions. Each solution has the form
[R(n,k), [a_0, a_1, ..., a_d]]
Zeilberger returns [] if it fails to find a solution.
Zeilberger invokes Gosper only if gosper_in_zeilberger is true.