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


DEBUG:Going to execute:
CoreTactics.block_goal
Level 4: evaluation returns
idtac of type tacvalue

Level 2: evaluation returns
idtac of type tacvalue

Level 1: evaluation returns
idtac of type tacvalue