When I change the STIMULUS process to this one:

STIMULUS: process
begin
a <= '0';
wait for 0 ns;

a <= '1' after 2 ns;
a <= '1' after 4 ns;
wait for 20 ns;

wait;
end process;

It seems all transaction executed. The waveform is '0' in (0-2ns) and
'1' after 2 ns.

Why are this transactions (all) executed?

a <= '1' after 2 ns;
a <= '1' after 4 ns;

I tested it with Aldec 5.1 simulator.


Thanks for any help
Attila