def sample(self, rvec0, nsteps, nskip=1):
        r"""generate nsteps sample points"""
        rvec = np.zeros(shape=(nsteps + 1, self.dim))
        rvec[0, :] = rvec0[:]
        for t in range(nsteps):
            q = rvec[t, :]
            for s in range(nskip):
                q = self.step(q)
            rvec[t + 1, :] = q[:]
        return rvec