I wonder why gs-quant/gs_quant/timeseries/statistics.py contains the SIR compartmental model class for infectious disease transmission, as well as the SEIR mode

I wonder why gs-quant/gs_quant/timeseries/statistics.py contains the SIR compartmental model class for infectious disease transmission, as well as the SEIR model (https://en.wikipedia.org/wiki/Compartmental_models_in_epidemiology https://en.wikipedia.org/wiki/Compartmental_models_in_epidem...)...