Louvain code getvid

GSQL Queries
1

Hi everyone,

I’m reading Louvain algorithm gsql and wonder why in this line using condition getvid(v) != -1. Why can internal Id equal -1?

      Start = SELECT s 
              FROM Start:s
              ACCUM
                  FOREACH v IN s.@cc_list DO
                      CASE WHEN getvid(v) != -1 THEN 
                          v.@cm_list += s 
                      END
                  END;