public extension PureYAML.Validation {
    /// A tree node a `Validator` can walk. `Validator`'s traversal engine is shared by every layer
    /// (validation-rules.md, principle 10); each `Traversable` conformance supplies only that layer's
    /// own traversal shape: "each layer defines only its own document, its rules, and its traversal".
    protocol Traversable: Validatable {
        /// Extra values to offer to type-directed checks at this node, beyond the node itself (e.g. a
        /// mapping's own type, or a mapping's key nodes). A `nil` path component offers at this node's
        /// own path; a non-nil one offers at that child's path (e.g. a mapping key's location).
        var extraOffers: [(component: Path.Component?, value: any Validatable)] { get }

        /// This node's children to recurse into, each paired with the path component that reaches it
        /// from this node.
        var children: [(component: Path.Component, value: Self)] { get }
    }
}