Goal:
  
  A : Type
  a : A
  l : list (option A)
  l' : list A
  l0 : list A
  Heq : map_option_out l = Some l0
  IHl : forall l' : list A, Some l0 = Some l' -> #|l| = #|l'|
  ============================
   (S #|l0| = #|a :: l0|)


DEBUG:Going to execute:
trivial; hnf; intros