Goal:
  
  A : Type
  l : list A
  P : A → bool
  n : nat
  d : A
  H : is_true (forallb P l)
  HH : #|l| ≤ n
  ============================
   (d = d)


DEBUG:Going to execute:
solve
[ trivial
with eq_true
| reflexivity
| symmetry; trivial
| contradiction
| congruence ]