Goal:
  
  A : Type
  P : A → A → Type
  Q : A → A → Type
  hd : A
  tl : list A
  tl' : list A
  X : OnOne2 P tl tl'
  IHX : (∀ x y : A, P x y → Q x y) → OnOne2 Q tl tl'
  X0 : ∀ x y : A, P x y → Q x y
  ============================
   (OnOne2 Q tl tl')


DEBUG:Going to execute:
assert X2
Level 1: evaluation returns
assert X2; [ exact (id0 id1) | clear id0 ]
where
id1 := X0
of type
intropattern
id0 := IHX
of type
intropattern
flags := 
of type
tauto_flags
X2 := OnOne2 Q tl tl'
of type
constr of type tacvalue