typeclass Copy =
  & copy : Copy -> Copy

typeclass (Ord : #Eq) =
  & is_less_then : Ord -> Bool

typeclass (D : #A #B #C) 'A 'B =
  & do_something : Unit -> (& 'A & 'B)

typeclass E 'A =
  & do_something : Unit -> 'A

decl ( == ) ('A : #Ord) : 'A -> 'A -> Bool
def ( == ) : a b = a.is_equal_to: b

decl ( < ) ('A : #Ord) : 'A -> 'A -> Bool
def ( < ) : a b = a.is_less_then: b

decl ( > ) ('A : #Ord) : 'A -> 'A -> Bool
def ( > ) : a b = !(a <= b)

decl ( <= ) ('A : #Ord) : 'A -> 'A -> Bool
def ( <= ) : a b = (a < b) || (a == b)

decl ( >= ) ('A : #Ord) : 'A -> 'A -> Bool
def ( >= ) : a b = !(a < b)