Creates a new property set containing the properties in this one,
            plus the ones of the property set passed as argument.

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]