Function probe 

pub fn probe<X, T, E>(
    start: X,
    most: usize,
    next: impl FnMut(&X) -> Result<X, E>,
    at: impl FnMut(&X) -> Result<Probe<T>, E>,
) -> Result<Option<T>, E>

Asks at for start and each value next makes of the one before, at most most of them, until one is Probe::Found or Probe::Stop. next is asked only for a value that will be probed. at may keep state of its own (what the probes so far said), since every value is asked once, in order.