(In fact, there's a fifth category, that's even rarer: multiple people assuming the same made-up identity. The only examples that come to mind are Alan Smithee, the name used by directors who aren't happy with the final version of a movie they've directed; Karen Eliot, the name used by anyone who wants to use it in the Neoist art movement; and a few mystery-writer identities that have been filled by a number of people over the years, like Carolyn Keene and Ellery Queen.)
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