Goal:
  
  A : Type
  B : Type
  R : A → B → Prop
  x : A
  y : list B
  a1' : list A
  a1 : list B
  a : Forall2 R a1' a1
  ============================
   ({| pr1 := [x]; pr2 := y |} = {| pr1 := a1'; pr2 := a1 |}
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in ∃ y' : B, y = [y'] ∧ R x y')


DEBUG:Going to execute:
fun p => first
             [ match p with
               | _ => generalize dependent p; intros p
               end
             | intros
             until
             p
             | intros
             until
             1
             | intros ]