Function less_than 

pub fn less_than<A, B, T, Fa, Fb>(
    left: Fa,
    right: Fb,
) -> LessThanJoiner<Fa, Fb, T>
where
    T: Ord,
    Fa: Fn(&A) -> T + Send + Sync,
    Fb: Fn(&B) -> T + Send + Sync,