[sv-ac] Sampled value function $past $rose $fell

Hi,
 Please consider this sequence and property.

sequence sq1;
@(posedge clk)  a ##1 $past(b,2, ,clk1);
endsequence

property prop;
@(posedge clk)##1 sq1;
endproperty

assert1: assert property(prop)

$past has clk1 in his argument , but the sequence or property has clk. 
Both this clk and clk1 are different.
My question is that while sampling which clock should $past take, clk or 
clk1? In place of $past, if $rose or $fell used then what will be the clock?

Thanks,
-Soumya


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