Macro rlp_encodable 

macro_rules! rlp_encodable {
    (
        $(#[$attr:meta])*
        $vis:vis struct $name:ident {
            $(
                $(#[$field_attr:meta])*
                $field_vis:vis $field_name:ident: $field_type:ty $(=> $cast:ty)?,
            )*
        }
    ) => { ... };
}

Create an RLP-encodable struct by specifying types to cast to.