Transform y with f using parameter vector psi
        psi = [[a,b,c]]

        :math:`f = (y * d) + \\sum_{terms} a * tanh(b *(y + c))`

def f(self, y):
        """
        Transform y with f using parameter vector psi
        psi = [[a,b,c]]

        :math:`f = (y * d) + \\sum_{terms} a * tanh(b *(y + c))`
        """
        d = self.d
        mpsi = self.psi
        z = d * y.copy()
        for i in range(len(mpsi)):
            a, b, c = mpsi[i]
            z += a * np.tanh(b * (y + c))
        return z