Goal:
    A : Type  B : Type  R : A → B → Prop  x : A  y0 : B  H : R x y0
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in ∃ y' : B, [y0] = [y'] ∧ R x y')


DEBUG:Going to execute:
change T; CoreTactics.intros_until_block
Level 3: T
 has value 
(let H := CoreTactics.block in ∃ y' : B, [y0] = [y'] ∧ R x y')