Goal:
  
  A : Type
  P : list A → A → Type
  x : A
  a1 : list A
  a2 : (λ index : list A, All_fold P index) a1
  ============================
   ([x] = a1
    → let H := CoreTactics.block in let H0 := CoreTactics.block in P [] x)


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