this bug is related to http://ekpbelle2.physik.uni-karlsruhe.de/redmine/issues/564

the attached graph shows the momentum distribution for secondary pions from decayed kaons, showing a stark difference between pi+ (black) and pi- (red). the decay rate also appears to be smaller for pi- (4018 events) than for pi+ (4268 events) - with the same amount of primary k+ and k- (10024 and 10028).