It preserves the location of the local declaration.
@Kha This tactic is going to be used to fix another hygiene related bug in
Lean3 :)
Here is small repro for the problem.
```
example (m n k : ℕ) (h : 0 + n = m) (h : k = m) : ... :=
begin
-- Here `h : k = m` is accessible.
rw [nat.zero_add] at *
-- `h : k = m` is not accessible anymore, and it is a name for
-- the simplified `h : n = m` which was inaccessible before.
end
```
