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:
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