constant for empty prefix set

[Alexander Zubkov]

Hi,

I propose to add some constant for empty prefix set, for example (as
in the patch):
define pfx_empty = [ /empty/ ];

It is useful when we define a named prefix list, which we can use
somewhere in filters, but this prefix list is generated, for example,
and we may need to make it empty. But there is no currently means to
create an empty prefix set, as I understand.
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