  The spectral decomposition of regular sl_2-invariant R-matrices R(lambda) is
studied by means of the method of reduction of the Yang-Baxter equation onto
subspaces of a given spin. Restrictions on the possible structure of several
highest coefficients in the spectral decomposition are derived. The origin and
structure of the exceptional solution in the case of spin s=3 are explained.
Analogous analysis is performed for constant R--matrices. In particular, it is
shown that the permutation matrix P is a ``rigid'' solution.
