See issue #1694.
There is an orthogonal issue. `simp` (and consequently `unfold`) cannot be used to
reduce projections (e.g., `has_add.add`). This issue has been
previously raised by @Armael, but it was not addressed yet.
closes#1675
After this commit, the following example works as expected.
```
example (p : nat → Prop) (a b : nat) : a = 0 ∧ b = 0 → p (a + b) → p 0 :=
begin
intros h₁ h₂,
simp [h₁] at *,
/- produces the state
(p : nat → Prop) (a b : nat)
h₁ : true
h₂ : p 0
|- p 0
-/
assumption
end
```
as expected.
Remark: the original issue raised by issue #1675 is actually solved by the
`simp_all` tactic.
