Possible wrong state space model formula

[mzaffalon]

$x^+$ is not a standard notation for the first derivative of x in https://baggepinnen.github.io/ControlSystemIdentification.jl/stable/ss/:

[baggepinnen]

$x^+$ does not denote the first derivative here, it denotes "$x$ in the next time instance", since this is a discrete-time model. The more verbose way of expressing this would be $x(k+1) = Ax(k) + Bu(k)$ but this is commonly abbreviated to $x^+$