@memoize()<EOL><INDENT>def _kurtosis(self, x, z, d=<NUM_LIT:0>):<DEDENT>
val = self._poly(z, self._kurtosis_coeffs(d))<EOL>return (np.tanh(val)+<NUM_LIT:1>)/<NUM_LIT>*(<NUM_LIT:3> - <NUM_LIT:6>*x**<NUM_LIT:2> + x**<NUM_LIT:4>)<EOL>
returns the kurtosis parameter for direction d, d=0 is rho, d=1 is z