{- -----------------------------------------------------------------------------
These inverse functions are used for simplifying propositions using
concrete natural numbers.
----------------------------------------------------------------------------- -}

-- | Subtract two natural numbers.
minus :: Integer -> Integer -> Maybe Integer
minus x y = if x >= y then Just (x - y) else Nothing
minus :: Integer -> Integer -> Maybe Integer
minus x y = if x >= y then Just (x - y) else Nothing
minus x y = if x >= y then Just (x - y) else Nothing