प्रस्तावकीय तर्क (propositional logic) में निम्नलिखित में से कौन सा द्वि-निषेध विलोपन (double-negation elimination) नियम को दर्शाता है?

Which of the following logical expressions is NOT a tautology?

A Horn clause is a clause with ____________ positive literal.

In logic programming, a Horn clause containing exactly one positive literal and a non-empty antecedent (body) is called a: