Goal:
    A : Type  P : list A → A → Type  x : A  a : All_fold P [x]
  ============================
   (let H := CoreTactics.block in P [] x)


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