Add `patch.plist` to `self.plist`.

def combine(self, patch):
        """Add `patch.plist` to `self.plist`."""
        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)