Goal:
  
  A : Type
  B : Type
  a : A
  l : list A
  IHl : ∀ l' : list B, #|l| = #|l'| → Forall2 (λ (_ : A) (_ : B), True) l l'
  H : S #|l| = 0
  ============================
   (Forall2 (λ (_ : A) (_ : B), True) (a :: l) [])


DEBUG:Going to execute:
do
![ solve
 [ trivial | apply : eq_sym ; trivial ]
 | discriminate
 | contradiction
 | Ltac.split ]