let prob_uwv ?(u = "") ?(v = "") w =
    match u,v with
    | None, None -> (float_of_int(H.find wf w)) /. (float_of_int(_N +_V))
    | Some u, None -> (float_of_int(H.find wpf (u,w)) /. ((float_of_int(H.find wf u) +. float_of_int(_V))
    | None, Some v -> (float_of_int(H.find wf w) /. float_of_int(_N +_V)) *. (float_of_int(H.find wpf (w,v)) /. ((float_of_int(H.find wf w) +. float_of_int(_V))
    | Some u, Some v -> (float_of_int(H.find wpf (u,w)) /. ((float_of_int(H.find wf u) +. float_of_int(_V)) *. (float_of_int(H.find wpf (w,v)) /. ((float_of_int(H.find wf w) +. float_of_int(_V))