code: def combine(self, patch):
          
          exclusive = set(["config", "default", "mandatory", "presence",
                       "min-elements", "max-elements"])
          kws = set([s.keyword for s in self.plist]) & exclusive
          add = [n for n in patch.plist if n.keyword not in kws]
          self.plist.extend(add)
docs: Add `patch.plist` to `self.plist`.