inductive Expr
| Var : Nat → Expr
| Val : Nat → Expr
| Add : Expr → Expr → Expr

open Expr Nat

def mkExpr : Nat → Nat → Expr
| 0        v := Val v
| (succ n) v := Add (mkExpr n (v+1)) (mkExpr n (v-1))

def eval : Expr → Nat
| (Var x) := 0
| (Val v) := v
| (Add l r) := eval l + eval r

def main : IO UInt32 :=
IO.println (toString $ eval (mkExpr 26 1)) *>
pure 0