empty refid

[Daniel Nelson]

I'm working on a project that attempts to parse the output of `ntpq -p`, and
have heard a report that sometimes the refid is empty.  Here is the output I'm
seeing, the indention was mangled by the logging system, so I've attempted to
realign things below:

remote            refid     st t when poll reach delay offset jitter
==============================================================================
 0.ubuntu.pool.n .POOL.     16 p -      64   0  0.000   0.000 0.000
+198.1.1.1                  16 u 1048 1024 376 15.384  -0.538 0.943

My question is, what does an empty refid indicate and are there any tips for
machine parsing the output correctly given that the lines may have a differing
number of columns.