Goal:
  
  A : Type
  B : Type
  R : A → B → Prop
  P : A → Prop
  x : A
  y : B
  l : list A
  l' : list B
  H : R x y
  H0 : Forall2 R l l'
  IHForall2 : Forall P l → Forall2 (λ (x : A) (y : B), P x ∧ R x y) l l'
  H2 : P x
  H3 : Forall P l
  ============================
   (Forall2 (λ (x : A) (y : B), P x ∧ R x y) l l')


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