Function derive_pair 

pub fn derive_pair<A, B, U>(
    first: A,
    second: B,
    map: impl Fn(<A as Source>::Value, <B as Source>::Value) -> U + 'static,
) -> Memo<U>
where
    A: Source + 'static,
    B: Source + 'static,
    U: PartialEq + 'static,

[derive] over two sources, recomputed when either moves — a label that reads a level and whether it is charging, and has to follow both.