[sv-ac] $past question

In the syntax for $past,

           $past( expression1 [, number_of_ticks] [, expression2] [,
clocking_event])

Is "number_of_ticks" required to be static or can it be variable?  PSL
requires that it is static, which allows performance optimizations.  I
don't see this in the SVA LRM even after Mantis 1698.  I'm not sure if
it is an oversight or if people really want to allow variable numbers.
And if it is variable, is there an upper limit?

Lisa

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