code: def normalize(self):
          
          if not self.subfilters:
              # No sub-filters
              return None
  
          # New sub-filters list
          new_filters = []
          for subfilter in self.subfilters:
              # Normalize the sub-filter before storing it
              norm_filter = subfilter.normalize()
              if norm_filter is not None and norm_filter not in new_filters:
                  new_filters.append(norm_filter)
  
          # Update the instance
          self.subfilters = new_filters
  
          size = len(self.subfilters)
          if size > 1 or self.operator == NOT:
              # Normal filter or NOT
              # NOT is the only operator to accept 1 operand
              return self
  
          # Return the only child as the filter object
          return self.subfilters[0].normalize()
docs: Returns the first meaningful object in this filter.