<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def contourf(x, y, z, ax, **kwargs):
 """ Filled contour plot of 2d DataArray Wraps :func:`matplotlib:matplotlib.pyplot.contourf` """

    primitive = ax.contourf(x, y, z, **kwargs)
    return primitive