type FieldSelector =
    | FactIndex of int
    | Sum of FieldSelector * FieldSelector
    | ProdLeft of FieldSelector
    | ProdRight of FieldSelector
    | Rec

  type Fact = int[]

  type TokenElement =
    | TEFact of Fact
    | TELeft of TokenElement
    | TERight of TokenElement
    | TEProd of TokenElement * TokenElement

  let lookupTE (fieldSelector:FieldSelector) (tokenElement:TokenElement) =
    let rec loop lsel te =
      match lsel, te with
      | FactIndex index, TEFact ar -> ar.[index]
      | Sum(fl, _), TELeft tel -> loop fl tel
      | Sum(_, fr), TERight ter -> loop fr ter
      | ProdLeft fs, TEProd(teFst,_) -> loop fs teFst
      | ProdRight fs, TEProd(_,teSnd) -> loop fs teSnd
      | Rec, te -> loop fieldSelector te
    loop fieldSelector tokenElement

  let listTE = TELeft <| TEFact [|1; 2|]
  let listTE2 = TERight <| TEProd(TEFact [|1; 2|],TELeft <| TEFact [|2; 3|])