Hello,
I have created a manual-communities.conf file in the BIRD directory with function like:

function action_communities() {
  if ((206016,0,999) ~ bgp_large_community) then _reject("By action community");
}

And then tried to use this function in pre-export attribute. But got an error when executing pathvector generate:

INFO[0000] Starting Pathvector devel                    
FATA[0000] BIRD: BIRD validation error:
        export filter {
            action_communities();
            ^ syntax error, unexpected CF_SYM_UNDEFINED
            remove_private_asns();

If I add this function directly to config ASxxx_NAME_v6.conf and run birdc configure check I got answer Configuration OK

After some investigating I found that pathvector creates cache dir and check config files from this dir BUT it don't include/copy manual*.conf files from original BIRD directory. Therefore, pathvector considers that the function from the manual-communities.conf file does not exist
So, I wrote this patch which copy all manual*.conf from BIRD directory to cache directory and this solve the problem.