  Kinematic fitting is a well-established tool to improve jet energy and
invariant mass resolutions by fitting the measured values under constraints
(e.g. energy conservation). However, in the presence of substantial ISR and
Beamstrahlung, naive energy and (longitudinal) momentum constraints fail due to
the a priori unknown amount of undetected momentum carried away by collinear
photons. It is possible to take care of those two effects and thus obtain
significantly higher mass resolutions.
