code: def to_special_value(self, value):
          
          if value == self.spdx_namespace.none:
              return utils.SPDXNone()
          elif value == self.spdx_namespace.noassertion:
              return utils.NoAssert()
          elif value == self.spdx_namespace.unknown:
              return utils.UnKnown()
          else:
              return value
docs: Checks if value is a special SPDX value such as
          NONE, NOASSERTION or UNKNOWN if so returns proper model.
          else returns value