(VAR x xs ys)

(DATATYPES a = µX. < nil, dd(X,X) >
 )

(SIGNATURES
   @ :: a x a -> a
   flatten :: a -> a
 )

(RULES
  @(nil, xs)         -> xs
  @(dd(x, xs), ys) -> dd(x, @(xs, ys))

  flatten(nil) -> nil
  flatten(dd(x,xs)) -> @(x,flatten(xs))
)