Post-conditions not satisfied after lemma call

[Lilii13]

Hello Fstar community,

I have some issues in the following piece of code. The first assert is exactly the pre-condition of lemma_test and works fine.
Calling the lemma is fine as well, however the assert that comes right after it, which represents the post-condition of the lemma is not verified.

assert(forall e. memP e l' ==> (el_of (e)) = (el_of (ep)) \/ (el_of (e)) ``disjoint`` (el_of (ep)));

lemma_test ep i n r l';

assert(forall e. memP e (fct i n r l') ==> (el_of (e)) = (el_of(ep)) \/ (el_of (e)) ``disjoint`` (el_of (ep)));
fct is a recursive function and lemma_test is also recursive, my guess is that the result of the lemma is not unfolded enough, that's why the post-condition is not valid, or is it something else? any solution on how to solve this?

Thanks a lot!