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 G4BertiniElectroNuclearBuilder
 ------------------------------
 It includes gamma-nuclear, electron-nuclear and positron-nuclear
 processes.
 For gamma-nuclear, it uses Bertini (BERT) model for gamma below 6 GeV,
 and Quark-Gluon-String (QGS) model above 3 GeV.
 For electron-nuclear and positron-nuclear, it uses the equivalent photon
 approximation in which the incoming lepton generates a virtual photon, 
 and then the virtual photon is converted to a real photon. This real
 photon is handled by BERT if its energy is below 10 GeV; if it is above
 10 GeV, then the real photon is transformed into a (on-shell) pi0 and
 then handled by Fritiof (FTF) string model.

 
 G4EmExtraPhysics
 ----------------
 It uses G4BertiniElectroNuclearBuilder for gamma-nuclear, electron-nuclear
 and positron-nuclear.
 Moreover, it includes muon-nuclear process (for mu- and mu+), and 
 synchrotron process (either for electron & positron, or for all
 charged particles).
 The muon-nuclear process is treated similarly as for electrons and
 positrons (i.e. the equivalent photon approximation in which the incoming
 lepton generates a virtual photon, and then the virtual photon is converted
 to a real photon, which is handled by BERT below 10 GeV or by FTF as a pi0
 above 10 GeV).
 By default, gamma-nuclear, electron-nuclear, positron-nuclear, and
 muon-nuclear are switched on, whereas synchrotron process is switched off
 for all particles. It is however possible, at run time via macro commands,
 to change this default (see G4EmMessenger below).


 G4EmMessenger
 -------------
 Used by G4EmExtraPhysics to be able to switch on/off: 
 -  synchrotron radiation for electron and positron
    (note: electron and positron together, not individually)
 -  synchrotron radiation for all charged particles
    (note: all charged particles together, not individually)
 -  gamma-nuclear, electron-nuclear and positron-nuclear
    (note: all these three particles together, not individually)
 -  muon-nuclear
    (note: mu- and mu+ together, not individually)
 at run time, via macro commands.


 G4CoherentPairProductionPhysics
 -------------
 Used by G4CoherentPairProduction process (in parameterisations/channeling/)
 of coherent pair production by high energy gamma in an oriented crystal.
 - IMPORTANT: with default parameters (incoherent scattering switched off)
   can work in parallel with a standard physics list.
 - It is possible to activate incoherent scattering using
   ActivateIncoherentScattering() function.
   IMPORTANT: requires switching off gamma conversion in
   the physics list to not simulate it twice
 - IMPORTANT: the geometry and material crystal data are contained in
   G4ChannelingFastSimCrystalData which is automatically copied from
   G4ChannelingFastSimModel into G4CoherentPairProduction using also
   the default G4ChannelingFastSimModel and G4Region names ("ChannelingModel"
   and "Crystal", respectively), which can be changed using
   SetNameChannelingModel and SetNameG4Region, respectively,
   being G4CoherentPairProductionPhysics set functions.
 - IMPORTANT: G4ChannelingFastSimModel and its G4Region are required
   for G4ChannelingFastSimCrystalData. If not found, an exception will
   be generated.
 - The intrinsic parameters of G4CoherentPairProduction can be changed
   using G4CoherentPairProductionPhysics set functions:
      - cuts of the model:
        - SetLowEnergyLimit(G4double energy); default is 1 GeV
        - SetHighAngleLimit(G4double angle); default is 50 mrad
        - SetPPKineticEnergyCut(G4double kineticEnergyCut); default is 1 MeV
      - internal parameters of the model (!!! it is strongly UNRECOMMENDED to decrease them):
        - SetSamplingPairsNumber(G4int nPairs);
            //default is 150 (amount of sampling pairs for Baier-Katkov algorithm)
        - SetChargeParticleAngleFactor(G4double chargeParticleAngleFactor);
            //default is 4 (angular distribution parameter = 4/gamma)
        - void SetNTrajectorySteps(G4int nTrajectorySteps);
            //default is 250 (amount of trajectory steps for tracking of
            //sampling pair in Baier-Katkov algorithm).
