Goal:
  
  A : Type
  P : A → Type
  a : A
  X : All P [a]
  X0 := {| pr1 := [a]; pr2 := X |} : sigma (λ index : list A, All P index)
  X1 := [a] : list A
  X2 := X : (λ index : list A, All P index) X1
  ============================
   (let H := CoreTactics.block in let H0 := CoreTactics.block in P 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.