Goal:
  
  A : Type
  P : A → Prop
  a : A
  l : list A
  H : ∀ x : A, In x (a :: l) → P x
  IHl : (∀ x : A, In x l → P x) → Forall P l
  ============================
   (a = a ∨ In a l)


DEBUG:Going to execute:
assert X2
Level 1: evaluation returns
assert X2; [ exact (id0 id1) | clear id0 ]
where
id1 := a
of type
intropattern
id0 := P
of type
intropattern
flags := 
of type
tauto_flags
X2 := Prop
of type
constr of type tacvalue