Hello,

it is hard for me to understand why leafs.ml does not match with leafs.mli,
while the compilation of leafs.ml alone is successful.

leafs.ml: 
let rec leafs = function
    `Node (a, b) -> (leafs2 a) @ (leafs2 b)
  | `Var _ as l -> [l]
  | others -> [others]

let leafs2 : ([> `Node of 'a * 'a ] as 'a) -> 'a list
  = (leafs2 :> ([> `Node of 'a * 'a ] as 'a) -> 'a list)

leafs.mli:
val leafs2 : ([> `Node of 'a * 'a ] as 'a) -> 'a list

Best regards,
Keiko NAKATA