Bitter constraint, and sad occasion dear, Compels me to disturb your season due: For Lycidas is dead, dead ere his prime... Milton is lamenting, once again, that Milton has been compelled to begin writing.

因为亲友的惨遇，痛苦的重压，迫使我前来扰乱你正茂的年华：,利西达斯死了，死于峥嵘岁月。,弥尔顿再一次哀叹道，他是不得以写这首诗的。

耶鲁公开课 - 弥尔顿课程节选

They come from writing i prime and j prime in terms of i and j, then sticking it in an expression and identifying the new representation.

写出用 i 和 j 来表示 i' 和 j' 的式子,再代入分量的表达式,就能得到新的式子

耶鲁公开课 - 基础物理课程节选