pub fn sample_sequentially<F: CpuLogpFunc, R: Rng + ?Sized>(
    logp: F,
    settings: SamplerArgs,
    start: &[f64],
    draws: u64,
    chain: u64,
    rng: &mut R
) -> Result<impl Iterator<Item = Result<(Box<[f64]>, impl SampleStats), NutsError>>, NutsError>