combineAR function
Implementation
List<double> combineAR(List<double> phi, List<double> Phi, int s){
final non=<double>[1.0, ...phi.map((v)=>-v)]; final seas=_seasonalPoly(Phi, s, ar:true);
final c=_polyMul(non,seas); return [for (int k=1;k<c.length;k++) -c[k]];
}