* Most operators perform <<.def relative>> to their input. They examine each input title in turn and filter out any that don't match. Such steps narrow down the output of a run.
* <<.def Absolute>> operators ignore their input and generate an independent output instead. <<.olink title>> is an example: the result of `[title[A]title[B]]` is `B`. But the <<.olink field>> operator is relative, so the result of `[title[A]field:title[B]` is empty.
