In the so-called isobar ansatz the three-particle states can only be populated
via an interacting two-particle system (resonant or non-resonant), and a spectator.
Using this ansatz, we derive the isobar-spectator interaction such that the
three-body unitarity is ensured exactly.
Some of the final equations are similar to the old Amado/Aaron/Young appoach,
however do not rely on arguments which are valid for bound-state isobars only.
Possible extensions and applications are discussed as well.