Goal:
    A : Type  H : ReflectEq A  l2 : list A  l : list A
  ============================
   (let H := CoreTactics.block in
    is_prefix l []
    → is_prefix l l2 → is_prefix l (fst (fst (split_prefix [] l2))))


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