(fitteia's extended models library)
double CROSSR1(double f, double a, double tau, double f0)
double CROSSR1(double f, double a, double tau, double f0){
double af;
af= a*( tau/(1+pow(2*pi*(f-f0)*tau,2.0)) + tau/(1+pow(2*pi*(f+f0)*tau,2.0)));
return af;
}