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 T with
           | context [ CoreTactics.block ] =>
               change T; CoreTactics.intros_until_block
           end