type good
type bad
type okay

type ('a, 'b) incl =
  | Incl_good : (good, okay) incl
  | Incl_bad  : (bad, okay) incl

type _ tree =
  | Node  : 'a tree * 'b tree * ('a, 'c) incl * ('b, 'c) incl -> 'c tree
  | Leaf  : good tree
  | Stump : bad tree