derivative of g wrt pi and tau
function g1pitau extends Modelica.Icons.Function; input SI.Pressure p "pressure"; input SI.Temperature T "temperature (K)"; output Real pi "dimensionless pressure"; output Real tau "dimensionless temperature"; output Real gpi "dimensionless dervative of Gibbs function wrt pi"; output Real gtau "dimensionless dervative of Gibbs function wrt tau"; end g1pitau;