Hello! Is it possible to make a rewriting inside a let expression that in turn is inside an Inductive definition? More precisely, I am using the Metalib package where [=] represents set equality. I have an hypothesis of the form H: a [=] b, and I want to replace some occurrences of a for b (rewrite H) inside a let expression that, in turn, is inside an Inductive definition. My current proof context is:

```
Hfv : fv_nom u [=] fv_nom u'
============================
(let (z, _) := atom_fresh (union (fv_nom u) (union (fv_nom (n_abs y t1)) (singleton x))) in
n_abs z (subst_rec_fun (swap y z t1) u x)) =a
(let (z, _) := atom_fresh (union (fv_nom u') (union (fv_nom (n_abs y t1)) (singleton x))) in
n_abs z (subst_rec_fun (swap y z t1) u' x))
```

where =a is an inductive predicate. I want to replace (fv_nom u) for (fv_nom u’) in the goal. When I try to “rewrite Hfv” I get Error: build_signature: no constraint can apply on a dependent argument.

Thank you in advance!

Regards,

Flávio.