```
-- Problem 10.
-- The next question concerns writing a function that collects all of the propositional
-- variables in a Prop. You will write a function, vars, that takes a Prop as input and
-- produces a list of String as output. The type template for this function is below.
-- Review "HaskellForGrownups" slides and, in particular, "How to Write a Haskell Function"
-- slides. The smart way to do this problem is in a stepwise fashion. That is, (1) replace
-- a single undefined with what you think the correct definition is and, then, (2) load that
-- into ghci to see if it typechecks. Your answer may produce repeated entries. For example,
-- evaluating (vars prob6) with ghci may return ["a","b","c","a","b","c"].
vars :: Prop -> [String]
vars (Atom p) = undefined
vars (Not p) = undefined
vars (Imply p1 p2) = undefined
vars (And p1 p2) = undefined
```

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