Goal:
  
  T : Type -> Type
  M : Monad T
  A : Type
  P : nat -> A -> Type
  Q : A -> Type
  f : forall (n : nat) (x : A), ∥ Q x ∥ -> T (∥ P n x ∥)
  n : nat
  a : A
  l : list A
  allq : All Q (a :: l)
  allq0 := {| pr1 := a :: l; pr2 := allq |}
        : sigma (fun index : list A => All Q index)
  allq1 : list A
  allq2 : (fun index : list A => All Q index) allq1
  ============================
   (a :: l = allq1 ->
    let H := CoreTactics.block in let H0 := CoreTactics.block in Q a)


DEBUG:Going to execute:
<coq-core.plugins.ltac::clear@0> $1