extension PureYAML.Model.Value: PureYAML.Validation.Traversable {
    /// For a mapping: the mapping itself (a `Validation<Mapping, _>` fires on this), and each pair's
    /// key node at its own path (so a key-typed check can inspect it, e.g. "is this key a string").
    /// Nothing extra for any other case.
    public var extraOffers: [(component: PureYAML.Validation.Path.Component?, value: any PureYAML.Validation.Validatable)] {
        guard case let .mapping(mapping) = self else {
            return []
        }
        return [(nil, mapping)] + mapping.pairs.map { pair in
            (component: pair.keyNode.pathComponent, value: pair.keyNode)
        }
    }

    /// A mapping's children are its pairs' values, keyed by their key node's path component; a
    /// sequence's children are its values, keyed by index. A scalar has none.
    public var children: [(component: PureYAML.Validation.Path.Component, value: PureYAML.Model.Value)] {
        switch self {
        case let .mapping(mapping):
            mapping.pairs.map { pair in (component: pair.keyNode.pathComponent, value: pair.value) }
        case let .sequence(values):
            values.indices.map { index in (component: .index(index), value: values[index]) }
        case .null, .bool, .int, .double, .string:
            []
        }
    }
}