-- | Make a lambda.
lambda :: [Identifier] -> Expression -> Expression
lambda vs e
  | map Variable vs == [e] = e
  | otherwise              = Function vs e
lambda :: [Identifier] -> Expression -> Expression
lambda vs e
  | map Variable vs == [e] = e
  | otherwise              = Function vs e
lambda vs e
  | map Variable vs == [e] = e
  | otherwise              = Function vs e