%Error: t/t_assert_future_bad.v:18:31: Future sampled value function called outside property or sequence expression (IEEE 16.9.4)
                                     : ... note: In instance 't'
   18 |   else $display("Future=%0d", $future_gclk(a));
      |                               ^~~~~~~~~~~~
        ... See the manual at https://verilator.org/verilator_doc.html?v=latest for more assistance.
%Error: t/t_assert_future_bad.v:21:31: Future sampled value function called outside property or sequence expression (IEEE 16.9.4)
                                     : ... note: In instance 't'
   21 |   else $display("Future=%0d", $rising_gclk(a));
      |                               ^~~~~~~~~~~~
%Error: t/t_assert_future_bad.v:24:31: Future sampled value function called outside property or sequence expression (IEEE 16.9.4)
                                     : ... note: In instance 't'
   24 |   else $display("Future=%0d", $falling_gclk(a));
      |                               ^~~~~~~~~~~~~
%Error: t/t_assert_future_bad.v:27:31: Future sampled value function called outside property or sequence expression (IEEE 16.9.4)
                                     : ... note: In instance 't'
   27 |   else $display("Future=%0d", $steady_gclk(a));
      |                               ^~~~~~~~~~~~
%Error: t/t_assert_future_bad.v:30:31: Future sampled value function called outside property or sequence expression (IEEE 16.9.4)
                                     : ... note: In instance 't'
   30 |   else $display("Future=%0d", $changing_gclk(a));
      |                               ^~~~~~~~~~~~~~
%Error: Exiting due to