I use two fields (type and entrysubtype) in formatting a particular entrytype [explanation below, in case it triggers any thoughts]. In most cases these are the same; occasionally there would be reasons to have them set differently. It would be a painful waste of a user's time to require both to be entered in cases where they would be identical.
To simplify the handling in biblatex, it occurred to me that I could just use biber to set the entrysubtype field from the type field if the user had not entered an entrysubtype field. For that purpose I use the following (in biber 0.9.9)
<map map_overwrite="0"> <per_type>LEGISLATION</per_type> <map_step map_field_source="type" map_final="1"/> <map_step map_field_set="entrysubtype" map_origfieldval="1"/> </map>
This works as expected. The "final" in the first step, strictly unnecessary, makes sure it doesn't fuss over cases where there is no type field. The second step correctly adds an entrysubtype only when none has been set. But in those cases, although its behaviour is perfectly correct, it generates warnings, which propagate to the LaTeX run too, to the effect that it has not overwritten the entrysubtype field.
Other things being equal, I'd rather not warn users when the program is doing what it's supposed to. Is there a way to stop such warnings? (What I suppose I need is a map step which checks if an entrytype is already set and terminates processing if it is; but I can't think how to construct such a step, though that's probably because I'm a complete biber.conf novice.) Or is there something to worry about that I'm not aware of? (I realise I could do all of this by checking for undefined fields within biblatex, but I think my alternative solution will make for cleaner code.)
[Type is used to determine what should be printed (say "Decision") and entrytype is used to determine what index a particular source ends up in. In most cases a piece of legislation that calls itself a Decision is a Decision: occasionally it's something else, and should be indexed accordingly.]