Function relation 

pub fn relation<V: TowerValue>(
    rel: Rel,
    left: &V,
    right: &V,
    recurse: &dyn Fn(Rel, V, V) -> Result<V, String>,
) -> Option<Result<V, String>>

Apply rel when either operand is an exact type; None when neither is. The answer is a Bool, or a residue-dependent Bool when it depends on ⌈1/dt⌉; a comparison no theorem decides within the budget is a loud error.