Function minitt::check::subtype::check_subtype_sum

pub fn check_subtype_sum<Sub, Super>(
    index: u32, 
    tcs: TCS, 
    sub_tree: BTreeMap<String, Sub>, 
    super_tree: BTreeMap<String, Super>, 
    sub_tree_eval: impl Fn(Sub) -> Value, 
    super_tree_eval: impl Fn(Super) -> Value
) -> TCM<TCS>

I'm not sure if I should recursively check or not, because if there's recursive sum type, recursively check_subtype will cause stack-overflow. A bug report is expected to prove this to be false.