Map Parser's UserState type

[3Rafal]

Hello,
I've found a curious case, which I don't know how to solve. I wonder if I'm missing some obvious solution.

Let's say that I have a type

type UserState =
  { Sub1: Substate1
    Sub2: Substate2 }

And I have a parser of type

let parseWithSub1 : Parser<'a,Substate1> =
  // Implementation...

Is there a posibility to map this state, so that I can use it like

let parseWithUserState : Parser<'a,UserState> =
  mapUserState (fun x -> x.Sub1) parseWithSub

where mapUserState would be a hypothetical function of signature ('u1 -> 'u2) -> Parser<'a,'u1> -> Parser<'a,'u2>?