If they don't write them like ''Present Laughter'' anymore, at least they keep trying. For instance, Douglas Anderson has written this slight, cheerful, formula romantic comedy set in a Midwestern college town. The plot concerns Eddie Evans (Christopher Loomis), a flaky mathematics professor obsessed by formulas, and Anne Yoder (Kathleen Swan), the departmental secretary who unwarily undertakes to type Eddie's voluminous textbook manuscript.
Mr. Anderson calls his comedy ''Knucklebones,'' after an old game of chance played with the knucklebones of a sheep. One of Eddie's intensely propounded theories has to do with the single law of chance, which holds that wildly improbable events will never occur. Eddie is a sort of brilliant knucklehead.
The wacky events on the stage of the Nat Horne Musical Theater are more eccentric than either wild or improbable. They require the committed comic expertise of the acting quintet (including a white rabbit) directed by Jay Perry. Besides Miss Swan and the energetic Mr. Loomis, the human players are Jean Barker as the professor's bingo-playing mother, who raises rabbits for magicians, and Arch Johnson as one of her customers, with some family revelations up his sleeve.
Mr. Anderson enjoys mingling humorously extravagant rhetoric, sight gags, and puns (Mrs. Evans's career has been ''hare raising''). At its best, the enjoyment is infectious. At its least, the antic view enlivens those Midwestern groves of academe. Daniel Proett's messy kitchen setting, David Loveless's costumes, and Zack Zanolli's lighting all add to the comic equation.