Goal:
    A : Type  P : A → Type  a : A  p : P a  X : All P []
  ============================
   (let H := CoreTactics.block in let H0 := CoreTactics.block in P a)


DEBUG:Going to execute:
match goal with
           | |- let _ := CoreTactics.block in ?T =>
                 match T with
                 | context [ CoreTactics.block ] =>
                     change T; CoreTactics.intros_until_block
                 end
           | _ => CoreTactics.unblock_goal
           end