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   The Breit-Wigner propagator is used for the s-channel 
virtual particles. The next level menu defines the
prescription for substitution of the Breit-Wigner
propagator to the amplitude. The 'fixed width' or 'running
width' prescriptions generally speaking do not respect the 
U(1) gauge invariance of the amplitude ('Gauge invariance OFF')
for some classes of processes (e.g. the reactions with the
t-channel photon exchange). Their implementation can result
in a nonunitary energy behaviour of the cross section. 
The 'overall' prescription respects the U(1) gauge invariance 
('gauge invariance ON') but suppresses the amplitude in the 
vicinity of p^2=m^2.
   See the Manual for details.