In some cases we know the set of fields that may appear in the records
of a given type, even if they are not mandatory. The %allowed
special field is used to specify this restriction. The usage is:
%allowed: field1 field2 ... fieldN
The field names are separated by one or more blank chracters.
If there are more or one %allowed fields in a record
descriptor, all fields of all the records in the record set must be in
the union of %allowed, %mandatory and %key.
Otherwise an integrity error is raised.
Several %allowed fields can appear in the same record
descriptor. The set of allowed fields is the union of all the
entries.