Goal:
  
  A : Type
  p : A → bool
  q : A → bool
  hpq : p =1 q
  a : A
  l : list A
  IHl : forallb p l = forallb q l
  ============================
   (p a && forallb p l = q a && forallb q l)


DEBUG:Going to execute:
rewrite (hpq a) IHl