function ArcTan2(const Y, X: Extended): Extended;
begin
  if X = 0 then
  begin
    if Y = 0 then
      Result := 0.0
    else if Y > 0 then
      Result := Pi/2
    else if Y < 0 then
      Result := -Pi/2;
  end
  else
    Result := ArcTan(Y/X);
  if X < 0.0 then
    Result := Result + Pi;
  if Result > Pi then
    Result := Result - 2*Pi;
end;