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


DEBUG:Going to execute:
match T with
           | context [ CoreTactics.block ] =>
               change T; CoreTactics.intros_until_block
           end