Goal:
  
  A : Type
  a : A
  x : A
  l : list A
  IHl : 0 < S #|l| → Some a = nth_error (List.rev l ++ [a]) (#|l| - 0)
  ============================
   (S #|l| = #|l| + 1 - 0)


DEBUG:Going to execute:
idtac