[isabelle] Termination for ordinal-like datatypes


I was wondering if someone could help me figure out an easy way to prove termination for functions over ordinal-like datatypes. What I mean by this is datatypes T where one of the constructors for T takes a function that returns type T. The classic one is the ordinals:

datatype ord
  = Zero
  | Succ ord
  | Limit "(nat => ord)”

My question is, what is the easiest and best way to write termination proofs for recursive functions over the type ord? Are there some mechanisms already available, or do I have to define my own inductive proposition or something similar? It looks like all the default mechanisms for proving termination rely on types that satisfy “size”, which ord does not because of the Limit constructor.

For example, I want to define functions like these, which should be terminating:

function ord_plus :: "ord => ord => ord"
  "ord_plus x Zero = x"
  | "ord_plus x (Succ y) = Succ (ord_plus x y)"
  | "ord_plus x (Limit f) = Limit (\<lambda> n . ord_plus x (f n))"
  by pat_completeness auto

function ord_leq :: "ord ⇒ ord ⇒ bool"
  "ord_leq Zero x = True"
  | "ord_leq (Succ x) Zero = False" 
  | "ord_leq (Succ x) (Succ y) = ord_leq x y"
  | "ord_leq (Succ x) (Limit f) = (\<exists> n. ord_leq (Succ x) (f n))"
  | "ord_leq (Limit f) y = (\<forall>n . ord_leq (f n) y)"
  by pat_completeness auto

Thanks very much for any help or suggestions.

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