def METHOD_NAME(self):
    """Check a *sufficient condition* that the expression is PSD,
    by checking if both arguments are PSD or both are NSD.
    """
    case1 = self.args[0].METHOD_NAME() and self.args[1].METHOD_NAME()
    case2 = self.args[0].is_nsd() and self.args[1].is_nsd()
    return case1 or case2