Goal:
  
  A : Type
  P : A → Type
  a : A
  X1 : list A
  X2 : (λ index : list A, All P index) X1
  ============================
   ([a] = X1
    → let H := CoreTactics.block in let H0 := CoreTactics.block in P a)


DEBUG:Going to execute:
try unfold signature in id'2