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:
fun id0 id1 X2 => assert X2; [ exact (id0 id1) | clear id0 ]