A Horn clause with exactly one positive literal is a definite clause; a definite clause with no negative literals is sometimes called a “fact”; and a Horn clause without a positive literal is sometimes called a goal clause. These three kinds of Horn clauses are shown in the following propositional example:
ʌ WORD_BOUNDARY h ɔ ɹ n WORD_BOUNDARY k l ɔ z WORD_BOUNDARY w ɪ ð WORD_BOUNDARY ɛ ɡ z æ k t l i WORD_BOUNDARY w ʌ n WORD_BOUNDARY p ɑ z ɪ t ɪ v WORD_BOUNDARY l ɪ t ə ɹ ɹ ə l WORD_BOUNDARY ɪ z WORD_BOUNDARY ʌ WORD_BOUNDARY d ɛ f ɪ n ə t WORD_BOUNDARY k l ɔ z WORD_BOUNDARY ʌ WORD_BOUNDARY d ɛ f ɪ n ə t WORD_BOUNDARY k l ɔ z WORD_BOUNDARY w ɪ ð WORD_BOUNDARY n oʊ WORD_BOUNDARY n ɛ ɡ ə t ɪ v WORD_BOUNDARY l ɪ t ə ɹ ɹ ə l z WORD_BOUNDARY ɪ z WORD_BOUNDARY s ʌ m t aɪ m z WORD_BOUNDARY k ɔ l d WORD_BOUNDARY ʌ WORD_BOUNDARY f æ k t WORD_BOUNDARY æ n d WORD_BOUNDARY ʌ WORD_BOUNDARY h ɔ ɹ n WORD_BOUNDARY k l ɔ z WORD_BOUNDARY w ɪ ð aʊ t WORD_BOUNDARY ʌ WORD_BOUNDARY p ɑ z ɪ t ɪ v WORD_BOUNDARY l ɪ t ə ɹ ɹ ə l WORD_BOUNDARY ɪ z WORD_BOUNDARY s ʌ m t aɪ m z WORD_BOUNDARY k ɔ l d WORD_BOUNDARY ʌ WORD_BOUNDARY ɡ oʊ l WORD_BOUNDARY k l ɔ z WORD_BOUNDARY ð iː z WORD_BOUNDARY θ ɹ iː WORD_BOUNDARY k aɪ n d z WORD_BOUNDARY ʌ v WORD_BOUNDARY h ɔ ɹ n WORD_BOUNDARY k l ɔ z ɪ z WORD_BOUNDARY ɑ ɹ WORD_BOUNDARY ʃ oʊ n WORD_BOUNDARY ɪ n ð ə WORD_BOUNDARY f ɑ l oʊ ɪ ŋ WORD_BOUNDARY p ɹ ɑ p ə z ɪ ʃ ə n ə l WORD_BOUNDARY ɛ ɡ z æ m p ə l WORD_BOUNDARY