extension PureYAML.Tagged {
    /// Key identity for duplicate detection: the resolved scalar value plus the
    /// key's effective tag (its carried tag, or the implicit tag of the resolved
    /// value). Two keys are the same key iff both match. Using the resolved value
    /// rather than the source spelling makes `0x1` and `1` (both `int(1)`)
    /// collide while `1` (`int`) and `"1"` (`str`) do not, and keeps
    /// custom-tagged keys (`!foo 1` vs `!bar 1`) distinct.
    struct MappingKeyIdentity: Hashable {
        var value: PureYAML.Model.Value
        var tag: String?
    }
}