extension PureYAML.Parsing.TokenEventParser {
    /// The key node of an explicit (`?`) mapping entry. The key may be empty: `? : v`, a trailing
    /// `? }` / `? ]` / `? ,`, or `?` at stream end all denote a null key. With no node and no
    /// properties to anchor, emit a null scalar rather than failing to find a node.
    mutating func parseExplicitMappingKey(
        properties: PureYAML.Parsing.NodeProperties,
    ) throws -> PureYAML.Parsing.NodeResult {
        guard properties == .none, isAtEmptyExplicitKey else {
            return try parseNode(properties: properties)
        }
        let mark = cursor.current?.mark ?? cursor.previous?.endMark ?? .start
        try emit(.scalar(value: "", anchor: nil, tag: nil, style: .plain, mark: mark, endMark: mark))
        return PureYAML.Parsing.NodeResult(kind: .scalar, endMark: mark)
    }

    /// Whether an explicit key has no key node: the next token is the value indicator, a flow
    /// terminator (`}`, `]`, `,`), or the stream has ended.
    var isAtEmptyExplicitKey: Bool {
        switch cursor.current?.kind {
        case nil, .mappingValue, .flowMappingEnd, .flowSequenceEnd, .flowEntry:
            true
        default:
            false
        }
    }
}