@Kha We will probably have to refine the heuristic for hiding the
inaccessible names, but the first version is already useful.
Here is the error message for a `_` before this commit
```
error: don't know how to synthesize placeholder
context:
x✝⁴ : List Nat
x✝³ : Nat
x✝² : x✝⁴ ≠ []
a b x✝¹ : Nat
x✝ : [a, b] ≠ []
⊢ Nat
```
After
```
error: don't know how to synthesize placeholder
a b : Nat
: [a, b] ≠ []
⊢ Nat
```
