variant was first broadly adopted at the 1978 World Science Fiction Convention in Phoenix, Arizona (IguanaCon) and is credited to Cary Hammer and Mark Malamud. Xghost (sometimes also known as Superduperghost or Llama) is played by adding a letter anywhere in the growing word fragment, including between letters. For example, given the fragment ERA, a player might offer BERA, ERAD, EBRA, or ERMA. This version was invented by Daniel Asimov around 1970. Originally and still often known as Superduperghost, it was played by his circle of mathematics grad student friends at U.C. Berkeley. Anaghost allows the player to rearrange (anagram) the letters in addition to adding one. For example, given the fragment ERA, a player might offer EART, EBAR, or NREA. Spook is played by adding letters to a "pool" in which no fixed order is assumed. In this game, one's objective is to avoid completing a letter pool which can be ordered to form a word. For example, given the pool {A,B,F,L,S,U}, a player would be unwise to add H, which would form the word BASHFUL. However, they might add B, and cite the word FLASHBULB if challenged. These variants usually require much more effort and time to play than the conventional game, and as such are lesser-known and less popular. Cheddar Gorge is played by adding a word to the end of a growing sentence fragment, and avoiding the completion of a sentence. This variant was popularized on the BBC Radio show I'm Sorry I Haven't a Clue'' History The name "ghost" is shortened from the original name "three thirds of a ghost"; a player, upon losing, became one, two, and finally three "thirds of a ghost", at which point they would float away and be out of the game. Computational complexity Given a regular expression R, if two players take turns playing Ghost with the language generated by R, the problem of determining whether player 1 has a winning strategy is in EXPSPACE, and is PSPACE-hard. It's proved to be PSPACE-hard by reducing Generalized Geography, a problem known to be PSPACE-hard, to a game of Ghost. Specifically, given a Generalized Geography graph, a nondeterministic finite automaton can be constructed, which gives a regular expression R, such that player 1 has a winning strategy in Ghost with R if and only if they have a winning strategy in the Generalized Geography game. This proof extends to Superghost, Superduperghost,