please use python to slove this question:

Write a function van_der_pol which takes input parameters mu, x0, and tf and plots the solution of the Van der Pol equation in the phase plane (ie. plots the trajectory x(t) versus x ' (t) in the x-x ' plane). The Van der Pol equation is :

d^2*x/ dt^2 (1 x^2 )* dx/dt + x = 0

with parameter defined by mu, initial conditions given by x0 (a Python list of length 2), and the plot is for t in the interval [0, tf ] (where tf is given by tf).