sml - How to get predecessor of a church numeral -

I am practicing with SML and I am doing a small assignment where we have to implement church numbers:

  datatype 'one church = c (' a -> 'a) *' a -> For example  

example

  zero = c (fn (f, x) => x)  

< P> I've already implemented the function:

  create: int -> 'Church Church: One Church - & gt; Int  

and SUC which gives the heir to the church number.

Now I have to implement the function

  PRED: 'a church - & gt; 'A church *' a church  

that gives the return of Tupal (predecessor, current number). I do not have permission to use churchToInt , I should work directly with church numbers, clearly it can be solved in a line by giving a specific argument in a row.

I was just thinking of using more than SUC until we hit the correct number but there is no way for me to compare 2 church points I'm totally stuck on this. Assume that you want to create a function that you n