Goal:
  
  A : Type
  B : Type
  P : B → A → A → Type
  Q : A → Type
  HPQ : ∀ (i : B) (x y : A), Q x → P i x y → Q y
  b : B
  bs : list B
  hd : A
  hd' : A
  tl : list A
  p : P b hd hd'
  e : #|bs| = #|tl|
  q : Q hd
  H : All Q tl
  ============================
   (P ?i hd hd')


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