[sv-ac] Mantis 1698

Hi to start the discussion, I copy here the note I put on mantis 1698
quite a while ago. Could that be used?

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Regardless where $past(x,,,@clk) is called from, the RTL equivalent is

always @clk past_variable <= $sampled(x);

and executes in the active region.

If $past is called from active, reactive or observed regions, replace
the call by $sampled(past_variable).

In synthesized code which executes in the active region, use the
past_variable directly. Provided that the model follows synthesis rules
and is not racy, the behavior should be equivalent to that in the
original model using $past.

The other functions, $rose, $fell and $stable would follow. 

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Best...
ed

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