Function sesh::offer_either

pub fn offer_either<'a, S1, S2, F, G, R>(
    s: Offer<S1, S2>, 
    f: F, 
    g: G
) -> Result<R, Box<dyn Error + 'a>> where
    S1: Session,
    S2: Session,
    F: FnOnce(S1) -> Result<R, Box<dyn Error + 'a>>,
    G: FnOnce(S2) -> Result<R, Box<dyn Error + 'a>>, 

Offer a choice between two sessions S1 and S2.