Organized authors write sequels. My stories probably require advanced math to keep track of (topology, maybe?). This is especially true when I get together with my friends and we combine worlds.
Mercedes Lackey and I wrote "Dragon in Distress" for SWORD & SORCERESS 12, which was published in July 1995. This means that we wrote it in 1994; the contract date is May 14, 1994. As I recall we did the original plotting in the hotel pool during the Fantasy Worlds Festival -- I remember us hanging on to a pair of kick-boards and moving back and forth along the pool as we talked (probably one of the few times authors actually got physical exercise while working). We used Rowena and the dragon from "The Birthday Gift" and Tarma and Kethry from Misty's "Vows & Honor" series and had their worlds collide. Then at the end of the story we sent Prince Florian into yet another world, thus setting him up to be a character in "A Prince Among Frogs" (which is a sequel to both "Weather Witch" and "Things That Go Grump in the Night").
"Dragon in Distress" is available for Kindle and Nook. It is also part of the collection THE PRINCESS, THE DRAGON, AND THE FROG PRINCE.