  In the framework of quantum groups and additive R-matrices, the fusion
procedure allows to construct higher-dimensional solutions of the Yang-Baxter
equation. These solutions lead to integrable one-dimensional spin-chain
Hamiltonians. Here fusion is shown to generalize naturally to non-additive
R-matrices, which therefore do not have a quantum group symmetry. This method
is then applied to the generalized Hubbard models. Although the resulting
integrable models are not as simple as the starting ones, the general structure
is that of two spin-(s times s') sl(2) models coupled at the free-fermion
point. An important issue is the probable lack of regular points which give
local Hamiltonians. This problem is related to the existence of second order
zeroes in the unitarity equation, and arises for the XX models of higher spins,
the building blocks of the Hubbard models. A possible connection between some
Lax operators L and R-matrices is noted.
