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
  allq : All Q (a :: l)
  allq0 := {| pr1 := a :: l; pr2 := allq |}
        : sigma (fun index : list A => All Q index)
  allq1 := a :: l : list A
  allq2 := allq : (fun index : list A => All Q index) allq1
  ============================
   (let H := CoreTactics.block in let H0 := CoreTactics.block in Q a)


DEBUG:Going to execute:
match goal with
           | id'1:=?t:_
             |- _ =>
                 generalize (eq_refl : t = id'1); clearbody id'2 id'1; clear
                  id' id; try unfold signature in id'2; 
                  hnf in id'2; simpl in id'2; rename id'2 into id;
                  cont id id id'1 t
           end
Level 1: Backtrace:

No head constant to reduce.