code: def parse_quality(self):
          
          if (self.quality is None):
              self.quality_val = self.default_quality
          elif (self.quality not in self.allowed_qualities):
              raise IIIFRequestError(
                  code=400, parameter="quality",
                  text="The quality parameter must be '%s', got '%s'." %
                  ("', '".join(self.allowed_qualities), self.quality))
          else:
              self.quality_val = self.quality
docs: Check quality paramater.
  
          Sets self.quality_val based on simple substitution of
          'native' for default. Checks for the three valid values
          else throws an IIIFRequestError.