<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def K_chol(self):
 """ Cholesky of the prior covariance K """

        if self._K_chol is None:
            self._K_chol = jitchol(self._K)
        return self._K_chol