Q. What is the rule for subject-verb agreement when a sentence has a collective noun + prepositional phrase + relative pronoun? For example, should it be “Scientists follow a set of guidelines that include x” (because the antecedent of that is guidelines), or “Scientists follow a set of guidelines that includes x” (because the subject is set)? Or does the answer differ depending on whether the writer wants to emphasize set or guidelines as the subject? And would the answer change if the sentence had “the set” instead of “a set” (as in the rule about mass noun + prepositional phrase)?
A. The verb goes with whichever noun is the subject. Often that will be clear {the box of rocks that was too wide for the door; the box of rocks that were brought back from space}. In your sentence the meaning is nearly the same whichever noun you choose, and that’s often true. The choice of article doesn’t matter {a box of rocks that was too wide; a box of rocks that were brought back from space}.