OCAML转发声明

有没有一种方法可以在OCAML中执行C样式的前向声明?

我的问题是,我有两个相互参照的变体:

type path_formula =
  [ `Next of state_formula
  | `Until of (state_formula * state_formula)
  | `UntilB of (state_formula * int * state_formula)  
  ]

type state_formula = 
    [ `True | `False
    | `Not of state_formula
    | `And of (state_formula * state_formula)
    | `Or of (state_formula * state_formula)
    | `Imply of (state_formula * state_formula)
    | `Label of string
    | `Prob` of (boundf * path_formula)
    | `Expc` of (boundi * formula)
    ]

所以两种类型都必须知道另一种。我在谷歌上搜索过,但不幸的是,ocaml并不是一种广泛使用的编程语言。