mkUnique actually is exported
GHC.Types.Unique
contains this code:
mkUnique :: Char -> Int -> Unique -- Builds a unique from pieces
-- NOT EXPORTED, so that we can see all the Chars that
-- are used in this one module
yet mkUnique
actually is exported (and apparently used profitably in other modules). I suppose I could just delete the comment, but I wanted to check if I should before doing so.