Mappings from ids to filters.
This is the filter we return in case no mapping was found for
given id; default is 'null' (in which case caller typically
reports an error), but can be set to an explicit filter.
Flag that indicates whether request for an unknown filter id should
result an exception (default) or not.
Note that this is only relevant if no default filter has been
mapping Mapping from id to filter; used as is, no copy is made.
Method for defining filter to return for "unknown" filters; cases
where there is no mapping from given id to an explicit filter.
f Filter to return when no filter is found for given id