vars a,b: Type

generated type Set a ::= empty | add a (Set a)

generated type Sys b ::= node b (Set (Sys b))

type Nat

var a: Type

generated type CTree a ::= leaf a | branch (Nat ->? CTree a)

free type L ::= abs (L -> L)

free type Tree a b ::= leaf b | branch (a -> Tree a b)

free type List a ::= nil | cons (a; List a)

free type Tree a b ::= leaf a | branch (b -> List (Tree a b))

var c : Type

op fold : (a -> c) -> ((b -> List c) -> c) -> Tree a b -> c

vars f : a -> c; g : (b -> List c) -> c; x : a; s : b -> List (Tree a b)

. fold f g (leaf x) = f x

. def (fold f g (branch s))