Goal:
  
  T : Type -> Type
  M : Monad T
  A : Type
  P : A -> Type
  Q : A -> Type
  f : forall x : A, ∥ Q x ∥ -> T (∥ P x ∥)
  l0 : list A
  a : A
  l : list A
  X : P a
  allq : All Q (a :: l)
  ============================
   (let H := CoreTactics.block in All Q l)


DEBUG:Going to execute:
fun tac => lazymatch goal with
                      | H:_ |- _ => tac H
                      end