Goal:
  
  A : Type
  p : A → bool
  x : A
  l : list A
  H : is_true (p x)
  H0 : Forall (λ x : A, p x) l
  IHForall : is_true (forallb p l)
  ============================
   (p x && forallb p l = true)


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