  Level repulsion is associated with exceptional points which are square root
singularities of the energies as functions of a (complex) interaction
parameter. This is also valid for resonance state energies. Using this concept
it is argued that level anti-crossing (crossing) must imply crossing
(anti-crossing) of the corresponding widths of the resonance states. Further,
itis shown that an encircling of an exceptional point induces a phase change of
one wave function but not of the other. An experimental setup is discussed
where this phase behaviour which differs from the one encountered at a diabolic
point can be observed.
