type BinTree =
    | Branch of BinTree * BinTree
    | None

let Leaf : BinTree = Branch(None, None)

(*
t1
 /\
/\ \
 /\
/

 t2
 /\
/
*)
let bintree2 = Branch(Branch(Leaf, None), Leaf)
let bintree1 = Branch(Branch(Leaf, bintree2), Branch(None, Leaf))