Log messages based on run-time tests

[Greg Harman]

Is there a best practice on generating run-time log entries based on  
data values (perhaps in the form of adding a row to another relation  
in the script)?

answer = FOREACH foo GENERATE id, bar;  -- If bar == null then log a  
warning.

The best I can think of uses FILTER, but this seems inefficient since  
I have to apply a filter to ANSWER instead of flagging the bad value  
as it came up in GENERATE:

warnings = FILTER answer BY (bar == null);