They don't have a permanent lawyer but the one they've hired a few times in Lionel Hutz.
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Phil Hartman. He tragically passed away several years ago, which
is why the character no longer appears on the show.
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First of all it's Selma not Thelma and she has been married to numerous men such as Troy McClure, Robert (Sideshow Bob) Terwilliger, Lionel Hutz, and Disco Stu, so now her name is Selma Bouvier-Terwilliger-Hutz-McClure-Stu.