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)
  ============================
   (let H := CoreTactics.block in let H0 := CoreTactics.block in P a)


DEBUG:Going to execute:
set (id'2 := pr2 id'); set (id'1 := pr1 id') in id'2;
            hnf in (value of id'1), (value of id'2);
            try red in (type of id'2)