  Most excited hadrons have multiparticle strong decay modes, which can often
be described as resulting from intermediate states containing one or two
resonances. In a theoretical approach, such a description in terms of
quasi-two-particle initial and final states leads to unitarity violations,
because of the complex masses of the involved resonances. In the present paper,
an empirical algebraic procedure is presented to restore unitarity of the
S-matrix while preserving its symmetry. Preliminary results are presented in a
first application to S-wave pion-pion scattering, in the framework of the
Resonance-Spectrum Expansion.
