Goal:
  
  X : Type
  L : list (option X)
  t : list X
  H : None = Some t
  IHL : ∀ t : list X,
          map_option_out L = Some t
          → All2 (λ (x : option X) (y : X), x = Some y) L t
  ============================
   (All2 (λ (x : option X) (y : X), x = Some y) (None :: L) t)


DEBUG:Going to execute:
inversion_clear H
Level 1: evaluation returns
inversion_clear H
where
H := H
of type
intropattern of type tacvalue