四分律行事鈔資持記 3.567

3.567

四中。引論因緣，聞者足以自照。化生蛇者，畜通四生故。延謂申之令長。四等即慈悲喜捨，平等慈攝，毒不能加，故用此觀。

In the fourth: citing the causes and conditions from the treatise is sufficient for listeners to reflect upon themselves. That [a poisonous thought] transformed into a serpent is because animals span the four types of birth. "Extend" means stretching it out to make it long. The "four equalities" are benevolence, compassion, joy, and equanimity; when grasped with equal loving-kindness, poison cannot harm one, which is why this contemplation is used.