extension PureYAML.Parsing {
    struct EventStreamContainer {
        var kind: EventStreamContainerKind
        var anchor: String?
        var values: [EventStreamComposedNode] = []
        var mergeSources: [[PureYAML.Model.Pair]] = []
        var localPairs: [PureYAML.Model.Pair] = []
        var pendingKey: PureYAML.Parsing.EventComposer.ComposedMappingKey?
        var mark: PureYAML.Parsing.Mark
        /// The running sum of every child's own expanded-node cost (see
        /// `EventStreamComposer.expandedNodeBudget`), so this container's total logical size --
        /// itself plus every descendant, alias expansion already accounted for -- is available in
        /// O(1) the moment it completes, without re-walking anything.
        var logicalSize = 0
        // Location tracking (parallel to the above, only when the composer tracks locations).
        var locatedValues: [PureYAML.Model.LocatedValue] = []
        var mergeLocatedSources: [[PureYAML.Model.LocatedPair]] = []
        var localLocatedPairs: [PureYAML.Model.LocatedPair] = []
        var pendingKeyNode: EventStreamComposedNode?
    }
}