gistfile1.swift

print("""
        <?xml version="1.0"?>
        <catalog>
        \(bookTuples.map(xmlForBookTuple).joined(separator: "")
        )</catalog>
        """)

typealias ChapterTuple = (heading: String)
typealias BookTuple = (id: String, author: String, title: String, genre: String, price: String, chapters: [ChapterTuple])
                
func xmlForChapterTuple(_ c: ChapterTuple) -> String {
    return """
                    <heading>\(heading)</heading>
        
        """
}
                
func xmlForBookTuple(_ b: BookTuple) -> String {
    return """
            <book id="bk\(b.id)" empty="">
                <author>\(b.author)</author>
                <title>\(b.title)</title>
                <genre>\(b.genre)</genre>
                <price>\(b.price)</price>
                <chapters>
            \(b.chapters.map(xmlForChapterTuple).joined(separator: "")
            )    </chapters>
            </book>
        
        """
}