Equation number 2 from:
  The International Association for the Properties of Water and Steam
  Gaithersburg, Maryland, USA
  September 2001
  Supplementary Release on  Backward Equations for Pressure as a
  Function of Enthalpy and Entropy p(h,s) to the IAPWS Industrial
  Formulation 1997 for the Thermodynamic Properties of Water and Steam
  
function h2ab_s extends Modelica.Icons.Function; output SI.SpecificEnthalpy h "Enthalpy"; input SI.SpecificEntropy s "Entropy"; end h2ab_s;