public extension PureXML.XPath {
    /// A compiled XPath 1.0 expression: all thirteen axes with their
    /// abbreviations, the node tests, the operator grammar (`or`, `and`, `=`,
    /// `!=`, `<`, `<=`, `>`, `>=`, `+`, `-`, `*`, `div`, `mod`, unary `-`, `|`),
    /// number and string literals, variable references, function calls, and
    /// filter expressions, all over the four-type model (node-set, boolean,
    /// number, string). Reimplemented from the XPath 1.0 specification.
    struct Query: Sendable {
        let expression: Expression

        /// Compiles an XPath expression, throwing ``QueryError`` on a syntax error.
        public init(_ path: String) throws {
            expression = try Compiler.compile(path)
        }

        /// Evaluates the query over a node, returning the selected node-set in
        /// document order. A non-node-set expression yields an empty list; use
        /// ``value(over:variables:)`` for typed results.
        public func evaluate(over node: PureXML.Model.Node) -> [Selection] {
            Evaluator.evaluate(expression, over: node)
        }

        /// Evaluates the query and returns the selected elements only.
        public func elements(over node: PureXML.Model.Node) -> [PureXML.Model.Element] {
            evaluate(over: node).compactMap(\.element)
        }

        /// Evaluates the query and returns the string-value of each selection.
        public func strings(over node: PureXML.Model.Node) -> [String] {
            evaluate(over: node).map(\.stringValue)
        }

        /// Evaluates the query to a typed ``Value`` with optional variable bindings.
        public func value(
            over node: PureXML.Model.Node,
            variables: [String: Value] = [:],
        ) throws -> Value {
            try Evaluator.value(expression, over: node, variables: variables)
        }

        /// Evaluates the query against an explicit context: a node already in a
        /// tree (``PureXML/Model/TreeNode``), its one-based proximity `position`
        /// within a node-set of `size`, and variable bindings. Downstream engines
        /// (XSLT, Schematron) drive this per context node; `position()` and
        /// `last()` reflect the supplied values.
        public func value(
            at node: PureXML.Model.TreeNode,
            position: Int = 1,
            size: Int = 1,
            variables: [String: Value] = [:],
        ) throws -> Value {
            try Evaluator.value(expression, at: node, position: position, size: size, variables: variables)
        }

        /// Evaluates the query and coerces the result to a number.
        public func number(over node: PureXML.Model.Node, variables: [String: Value] = [:]) throws -> Double {
            try value(over: node, variables: variables).number
        }

        /// Evaluates the query and coerces the result to a string.
        public func string(over node: PureXML.Model.Node, variables: [String: Value] = [:]) throws -> String {
            try value(over: node, variables: variables).string
        }

        /// Evaluates the query and coerces the result to a boolean.
        public func boolean(over node: PureXML.Model.Node, variables: [String: Value] = [:]) throws -> Bool {
            try value(over: node, variables: variables).boolean
        }
    }
}