entity tc4 is
end;

library ieee;
use ieee.std_logic_1164.all;

architecture behav of tc4 is
  signal clk : std_logic;
  signal tg : std_logic;
begin
  process (clk) is
  begin
    if falling_edge(clk) and tg then
      null;
    end if;
  end process;
end behav;