Goal:
  
  A : Type
  P : A → A → Type
  Q : A → A → Type
  x : A
  y : A
  l : list A
  l' : list A
  X : All2 (Trel_conj P Q) l l'
  a : P x y
  b : Q x y
  a0 : All2 P l l'
  b0 : All2 Q l l'
  ============================
   (All2 P l l')


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