Goal:
    A : Type  P : A → Type  a : A  p : P a  X : All P []
  ============================
   (let H := CoreTactics.block in P a)


DEBUG:Going to execute:
match goal with
           | |- let _ := CoreTactics.block in _ => intros _
           | |- _ => try (intro; CoreTactics.intros_until_block)
           end