Function fold_choice2

pub fn fold_choice2<Var, Foo>(list_len: usize, n: usize, f: Foo) -> Var
where
    Foo: Fn(usize, usize) -> Var,
    Var: Clone + Add<Var, Output = Var> + From<u64>,

A convenience helper: given a list_len and n (arguments of choice2), it creates an array consisting of f(i,j) where i,j \in [0,list_len] such that i + j = n, and then sums all the elements in this array.