  We apply the fusion procedure to a quantum Yang-Baxter algebra associated
with time-discrete integrable systems, notably integrable quantum mappings. We
present a general construction of higher-order quantum invariants for these
systems. As an important class of examples, we present the Yang-Baxter
structure of the Gel'fand-Dikii mapping hierarchy, that we have introduced in
previous papers, together with the corresponding explicit commuting family of
quantum invariants.
