Some writers choose to introduce elements or characters from one work into another, to present the idea that both works are set in the same universe. For example, the character of Ursula Buffay from American sitcom "Mad About You" was also a recurring guest star in "Friends", despite the two series having little else in common. Fellow NBC series "Seinfeld" also contained crossover references to "Mad About You". L. Frank Baum introduced the characters of Cap'n Bill and Trot (from "The Sea Fairies") into the Oz series in "The Scarecrow of Oz", and they made a number of appearances in later Oz books. In science fiction, A. Bertram Chandler introduced into his future Galactic civilization the character Dominic Flandry from Poul Anderson's quite different Galactic future (he had Anderson's consent)—on the assumption that these were two alternate history timelines and that people could on some occasions cross from one to the other.