Goal:
    A : Type  f : A → bool
  ============================
   (let H := CoreTactics.block in
    forallb_InP [] (λ (x : A) (_ : In x []), f x) = forallb f [])


DEBUG:Going to execute:
match goal with
           | hyp:context [ CoreTactics.block ]
             |- _ =>
                 cbn beta in hyp; eqns_specialize_eqs_block hyp;
                  cbn[eq_rect_r eq_rect] in hyp
           end