public extension PureYAML.Validation {
    /// Marker for a value a type-directed validation can be applied to. A `Validation<Subject>` fires on
    /// `Subject` values wherever they appear in the traversed tree; this protocol constrains the
    /// subjects so the erased dispatch can filter by runtime type.
    protocol Validatable: Sendable {}
}

extension PureYAML.Model.Value: PureYAML.Validation.Validatable {}
extension PureYAML.Model.Mapping: PureYAML.Validation.Validatable {}
extension PureYAML.Model.Key: PureYAML.Validation.Validatable {}
extension PureYAML.Tagged.Node: PureYAML.Validation.Validatable {}
extension PureYAML.Tagged.Mapping: PureYAML.Validation.Validatable {}