A famous line may be recognized at once by knowledgeable readers, and its familiarity can itself indicate an earlier source.
An obscure borrowed line ordinarily needs an explicit or contextual signal. The purpose of that indication is not to diminish the new poem, but to distinguish the new composition from the part supplied by another poet.
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Iqtibas, Tadmeen, Aqd, Hall, Talmih and the Art of Literary Openings and Endings
A famous line may be recognized at once by knowledgeable readers, and its familiarity can itself indicate an earlier source.
An obscure borrowed line ordinarily needs an explicit or contextual signal. The purpose of that indication is not to diminish the new poem, but to distinguish the new composition from the part supplied by another poet.