from G4Calo import GeometryDescriptor, G4System

cw = GeometryDescriptor()
for _ in range(25):
    cw.addLayer(0.5, "G4Pb", False)
    cw.addLayer(1.,"G4_POLYSTYRENE",True,1)

#directly use G4System as a global singleton
G4System.init(cw)