Goal:
  
  A : Type
  P : A → A → Type
  Q : A → Type
  hd : A
  tl : list A
  tl' : list A
  X : OnOne2 P tl tl'
  IHX : All Q tl → OnOne2 (λ x y : A, P x y × Q x) tl tl'
  X0 : Q hd
  X1 : All Q tl
  ============================
   (OnOne2 (λ x y : A, P x y × Q x) tl tl')


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