Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  a : A
  Γ : list A
  d' : A
  Γ'0 : list A
  Hrel : All2_fold P Γ Γ'0
  p : P Γ Γ'0 a d'
  ============================
   (Some d' = Some ?d' × All2_fold P Γ Γ'0 × P Γ Γ'0 a ?d')


DEBUG:Going to execute:
assert X2
Level 1: evaluation returns
assert X2; [ exact (id0 id1) | clear id0 ]
where
id1 := Γ'0
of type
intropattern
id0 := P
of type
intropattern
flags := 
of type
tauto_flags
X2 := list A → A → A → Type
of type
constr of type tacvalue