Text: While libertarians have endorsed giving the Postal Service to its employees, there don't appear to be any commercial calls for privatization (no doubt the $27 billion in government-guaranteed pension liabilities is a factor) or challenges to the USPS monopoly on first-class delivery.
Entailment: Libertarians have endorsed giving the Postal Service to its employees.
Contradiction: Libertarians haven't endorsed giving the Postal Service to its employees.