Function upper_central_length 

pub fn upper_central_length(
    degree: usize,
    gens: &[Perm],
    cap: usize,
) -> Option<usize>

The length of the upper central series when it reaches G (the nilpotency class) — None if it stalls below G (the group is not nilpotent) or |G| > cap. Equals nilpotency_class for nilpotent groups, an independent route to the same number.