type Tree<'T> =
        | Empty
        | Node of left:Tree<'T> * right:Tree<'T> * value:'T

    let isSymmetric (tree : Tree<'T>) =
        let rec loop (left : Tree<'T>, right : Tree<'T>) =
            match (left, right) with
                | (Node (lleft, lright, lval), Node (rleft, rright, rval)) ->
                    if lval = rval
                    then loop (lleft, rright) && loop (rleft, lright)
                    else false
                | (Empty, Empty) -> true
                | _ -> false
        match tree with
            | Node (left, right, _) -> loop (left, right)
            | Empty -> true