code: def validate(self):
          
          super(TaggedAsFilter, self).validate()
          self._value = self._value.lower()
  
          # If the tag starts with '=', test on equality (just this tag, no others)
          if self._value.startswith('='):
              self._exact = True
              self._value = self._value[1:]
          else:
              self._exact = not self._value
  
          # For exact matches, value is the set to compare to
          if self._exact:
              # Empty tag means empty set, not set of one empty string
              self._value = set((self._value,)) if self._value else set()
docs: Validate filter condition (template method).