bgp_next_hop check?

[Job Snijders]

Dear BIRD devs,

On https://gitlab.labs.nic.cz/labs/bird/wikis/BGP_filtering I see the
following curious snippet:

    function xx()
    {
        ...
        if bgp_next_hop != from then return false;
        ...
    }

What in the above example does "from" mean? Is "from" some kind of magic
keyword that expands into something?

Kind regards,

Job