Novels can remain unfinished because the author continually rewrites the story. When enough material exists, someone else can compile and combine the work, creating a finished story from several different drafts. Mark Twain's "The Mysterious Stranger" was written in three different versions over a period of 20 years, none of which were completed. Twain biographer and literary executor Albert Paine combined the stories and published his version six years after Twain's death. Similarly, J. R. R. Tolkien continuously rewrote "The Silmarillion" throughout his lifetime; a definitive version was still uncompiled at the time of his death, with some sections very fragmented. His son, Christopher Tolkien, invited fantasy fiction writer Guy Gavriel Kay to reconstruct some parts of the book, and they eventually published a final version in 1977. In 1980, Christopher Tolkien published another posthumous collection of his father's unfinished work, appropriately entitled "Unfinished Tales". Between 1982 and 1996, he published twelve volumes of "The History of Middle-earth", a substantial portion of which is unfinished and incomplete drafts. In 2007, Christopher Tolkien published another novel from his father entitled "The Children of Húrin". Like "The Silmarillion", Christopher assembled the novel from various incomplete drafts.