public extension PureXML.XSLT {
    /// One part of an attribute value template: literal text, or an embedded XPath
    /// expression written `{expr}`.
    enum ValuePart: Sendable {
        case literal(String)
        case expression(String)
    }

    /// An attribute value template (a string with `{expr}` substitutions).
    typealias ValueTemplate = [ValuePart]

    /// A literal-result-element attribute, whose value is an attribute value
    /// template.
    struct LiteralAttribute: Sendable {
        public var name: PureXML.Model.QualifiedName
        public var value: ValueTemplate
    }

    /// A sort specification for `apply-templates`/`for-each`.
    struct Sort: Sendable {
        public var select: String
        public var descending: Bool
        public var numeric: Bool
    }

    /// One instruction of a template body (a sequence constructor item).
    indirect enum Instruction: Sendable {
        /// Literal character data copied to the result.
        case literalText(String)
        /// `xsl:value-of`: the string value of an XPath expression.
        case valueOf(select: String)
        /// `xsl:apply-templates` with an optional node-set selection, mode, sorts,
        /// and passed parameters.
        case applyTemplates(select: String?, mode: String?, sorts: [Sort], parameters: [Binding])
        /// `xsl:for-each` over a node-set.
        case forEach(select: String, sorts: [Sort], body: [Instruction])
        /// `xsl:if`.
        case ifInstruction(test: String, body: [Instruction])
        /// `xsl:choose` with its `when` branches and optional `otherwise`.
        case choose(whens: [Branch], otherwise: [Instruction])
        /// A literal result element, copied to the output with its attributes.
        case literalElement(name: PureXML.Model.QualifiedName, attributes: [LiteralAttribute], body: [Instruction])
        /// `xsl:element` whose name is an attribute value template.
        case element(name: ValueTemplate, body: [Instruction])
        /// `xsl:attribute` whose name is an attribute value template.
        case attribute(name: ValueTemplate, body: [Instruction])
        /// `xsl:copy`: a shallow copy of the context node.
        case copy(body: [Instruction])
        /// `xsl:copy-of`: a deep copy of a selected node-set.
        case copyOf(select: String)
        /// `xsl:call-template` by name with passed parameters.
        case callTemplate(name: String, parameters: [Binding])
        /// `xsl:variable` bound for the rest of the sequence constructor.
        case variable(name: String, select: String?, body: [Instruction])
        /// `xsl:number`: a generated sequence number for the context node.
        case number(count: String?, from: String?, format: String)
        /// `xsl:comment`: a comment node whose text is the instantiated body.
        case comment(body: [Instruction])
        /// `xsl:processing-instruction` whose target is `name` and whose data is
        /// the instantiated body.
        case processingInstruction(name: ValueTemplate, body: [Instruction])
    }

    /// The `xsl:output` controls that shape serialization: the output method, and
    /// the declaration and indentation settings. Each is nil when unspecified, so
    /// the serializer falls back to the caller's options.
    struct Output: Sendable {
        public var method: String?
        public var indent: Bool?
        public var omitXMLDeclaration: Bool?
        public var encoding: String?
        public var version: String?
        public var standalone: Bool?

        public init(
            method: String? = nil,
            indent: Bool? = nil,
            omitXMLDeclaration: Bool? = nil,
            encoding: String? = nil,
            version: String? = nil,
            standalone: Bool? = nil,
        ) {
            self.method = method
            self.indent = indent
            self.omitXMLDeclaration = omitXMLDeclaration
            self.encoding = encoding
            self.version = version
            self.standalone = standalone
        }

        /// This output's settings with `other`'s non-nil settings layered over them.
        func merged(with other: Output) -> Output {
            Output(
                method: other.method ?? method,
                indent: other.indent ?? indent,
                omitXMLDeclaration: other.omitXMLDeclaration ?? omitXMLDeclaration,
                encoding: other.encoding ?? encoding,
                version: other.version ?? version,
                standalone: other.standalone ?? standalone,
            )
        }
    }

    /// An `xsl:key` declaration: a name, the nodes it indexes (a match pattern),
    /// and the key value of each (a `use` expression).
    struct Key: Sendable {
        public var name: String
        public var match: String
        public var use: String
    }

    /// One `xsl:when` branch of a choose.
    struct Branch: Sendable {
        public var test: String
        public var body: [Instruction]
    }

    /// A name binding: an `xsl:param` declaration (with a default) or an
    /// `xsl:with-param` value, supplied either by `select` or by a body.
    struct Binding: Sendable {
        public var name: String
        public var select: String?
        public var body: [Instruction]
    }

    /// A template rule: a match pattern and/or a name, a mode, a priority, its
    /// import precedence (higher wins, used to resolve `xsl:import`), its declared
    /// parameters, and a body.
    struct Template: Sendable {
        public var match: String?
        public var name: String?
        public var mode: String?
        public var priority: Double
        public var importPrecedence: Int
        public var parameters: [Binding]
        public var body: [Instruction]
    }

    /// A compiled stylesheet: its template rules, global variables, keys, and
    /// output controls (with `xsl:include`/`xsl:import` already folded in).
    struct Stylesheet: Sendable {
        public var templates: [Template]
        public var globals: [Instruction]
        public var keys: [Key]
        public var output: Output
    }
}