sorry, "i ≤ j” should be “i < j” actually:

   ∀i j. i < j ⇒ f (SUC i) ⊆ f j
   ------------------------------------
     0.  ∀n. f n ⊆ f (SUC n)
     1.  ∀n. 0 < n ⇒ f 0 ⊆ f n

> Il giorno 05 gen 2019, alle ore 19:32, Chun Tian (binghe) 
> <binghe.l...@gmail.com> ha scritto:
> 
> Hi,
> 
> I have the following goal to prove: (f : num -> ‘a set)
> 
>   ∀i j. i ≤ j ⇒ f (SUC i) ⊆ f j
>   ------------------------------------
>     0.  ∀n. f n ⊆ f (SUC n)
> 
> but how can I do the induction on … e.g. `j - i`?
> 
> —Chun
> 

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