Goal:
  
  A : Type
  l : list (option A)
  l' : list A
  IHl : forall l' : list A, map_option_out l = Some l' -> #|l| = #|l'|
  ============================
   (None = Some l' -> S #|l| = #|l'|)


DEBUG:Going to execute:
<coq-core.plugins.ltac::discriminate@0>