extension PureYAML.Parsing.LibYAML {
    /// Port of `yaml_simple_key_t`. A potential implicit ("simple") mapping key:
    /// a node that may turn out to be a key once a `:` is seen on the same line.
    /// `tokenNumber` records where its `KEY` token must be spliced into the queue.
    struct SimpleKey: Equatable {
        /// Whether a simple key is still possible at this flow level.
        var possible: Bool
        /// Whether a `:` is required (a key sitting exactly at the block indent).
        var required: Bool
        /// The absolute token number where the `KEY` token is inserted.
        var tokenNumber: Int
        /// The position the potential key started at.
        var mark: PureYAML.Parsing.Mark
        /// Whether this key is a JSON-style node (a quoted flow scalar). A `:`
        /// adjacent to such a key is a value indicator even without a following
        /// space, per the YAML flow "adjacent value" rule.
        var isJSON = false
        /// Whether a tab preceded this key in its indentation. A tab may be
        /// separation but never indentation, so promoting such a key to a block
        /// mapping key (a `:` follows) is an error.
        var tabbed = false

        /// The empty entry pushed for a new flow level (`{ 0, 0, 0, {0,0,0} }`).
        static var empty: Self {
            Self(possible: false, required: false, tokenNumber: 0, mark: .start)
        }
    }
}