Goal:
  
  A : Type
  B : Type
  P : A → B → Prop
  a : A
  l : list A
  f : Forall2 P (a :: l) []
  IHl : ∀ l' : list B, Forall2 P l l' → All2 P l l'
  ============================
   (All2 P (a :: l) [])


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