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 This menu function changes the form of the Breit-Wigner propagator.
For the case 'S-dependence OFF' the following expressions for its
scalar part are substituted:

a) Power=1                       b) Power=2

       P^2 - Mass^2                           1
 ----------------------------    ------------------------------
(P^2-Mass^2)^2 + (Mass*Width)^2  (P^2-Mass^2)^2 + (Mass*Width)^2

Here P, Mass, Width are the virtual particle Lorentz momentum,
mass and width correspondingly.
  For the case 'S-dependence  ON' the Width is replaced by

                    P^2   
Width -> Width * ---------
                   Mass^2

  This substitution simulates higher order corrections.