def _jtj(self, funct, params=None, dl=<NUM_LIT>, rts=False, **kwargs):
grad = self._grad(funct=funct, params=params, dl=dl, rts=rts, **kwargs)<EOL>return np.dot(grad, grad.T)<EOL>
jTj of a `func` wrt to parmaeters `params`. (see _graddoc)