entries (x:xs) = case Lexicon.lookup x of
      Just x' -> entries xs >>= Just . (x':)
      Nothing -> Nothing

--    entries xs >>= (Lexicon.lookup x:)

    -- Some often used formulae
entries (x:xs) = case Lexicon.lookup x of
      Just x' -> entries xs >>= Just . (x':)
      Nothing -> Nothing

--    entries xs >>= (Lexicon.lookup x:)

    -- Some often used formulae
entries (x:xs) = case Lexicon.lookup x of
      Just x' -> entries xs >>= Just . (x':)
      Nothing -> Nothing

--    entries xs >>= (Lexicon.lookup x:)

    -- Some often used formulae