type ReferenceState = enumeration(ASHRAE "0 enthalpy and entropy for saturated liquid at -40ºC", NBP "0 enthalpy and entropy for saturated liquid at normal boiling point", IIR "H=200 KJ/kg, and S=1.0 KJ/(kg·K) for saturated liquid at 0ºC", User "User defined reference_T and reference_P", None "No reference is used");