scan_tuple.fs

      fun (f : 'a * Structure -> 'a * Structure) (state : 'a) (list : Structure list) ->
        let g f (state,_) s = f (state, s)
        let states, structures = 
          list
          |> List.scan (g f) (state,Empty)
          |> List.unzip 
        // got to be a better way to pop the end off a list
        let final_state = states.[List.length states - 1]
        final_state, structures