<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add (self, ps):
 """ Creates a new property set containing the properties in this one, plus the ones of the property set passed as argument. """

        assert isinstance(ps, PropertySet)
        if ps not in self.added_:
            self.added_[ps] = create(self.all_ + ps.all())
        return self.added_[ps]