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


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