Function outcome 

pub fn outcome<O: Operation>(
    part: &NamedModel,
    args: &O::Args,
    is_refusal: fn(&str) -> bool,
) -> Outcome

Applies a step of O with args to a copy of part and says how it went: refused, where the error’s root message is one is_refusal accepts — the operation refusing what it does not support, rather than failing at what it does.