Goal:
  
  A : Type
  p : A → bool
  n : nat
  x : A
  HPl : is_true (forallb p [])
  Hnth : None = Some x
  ============================
   (p x = true)


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