datatype list a = Nil | Cons of a * list a fun isNil (t ::: Type) (ls : list t) : bool = case ls of Nil => True | Cons _ => False fun append (t ::: Type) (ls1 : list t) (ls2 : list t) : list t = case ls1 of Nil => ls2 | Cons (x, ls1') => Cons (x, append ls1' ls2) fun appendR (t ::: Type) (ls2 : list t) (ls1 : list t) : list t = case ls1 of Nil => ls2 | Cons (x, ls1') => Cons (x, appendR ls2 ls1') (*fun naughty (t ::: Type) (ls : list t) : list t = naughty ls*) fun append1 (t ::: Type) (ls1 : list t) (ls2 : list t) : list t = case ls1 of Nil => ls2 | Cons (x, ls1') => Cons (x, append2 ls2 ls1') and append2 (t ::: Type) (ls2 : list t) (ls1 : list t) : list t = case ls1 of Nil => ls2 | Cons (x, ls1') => Cons (x, append1 ls1' ls2)