[< 0 > + < 1 > * < 2 >]
     : nat
[< b > + < b > * < 2 >]
     : nat
[<< # 0 >>]
     : option nat
[1 {f 1}]
     : Expr
fun (x : nat) (y z : Expr) => [1 + y z + {f x}]
     : nat -> Expr -> Expr -> Expr
fun e : Expr =>
match e with
| [x y + z] => [x + y z]
| [1 + 1] => [1]
| _ => [e + e]
end
     : Expr -> Expr
[(1 + 1)]
     : Expr