“Shall” means “shall.” Unless it means “may.” And vice versa.
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Why is shorter likely to be more persuasive?

Mister Thorne explains why a shorter piece is more likely to hit the target than a longer one:

Very often, the shortest proposal has a distinct advantage over all others — people tend to read it first. And that’s usually because they’re pressed for time. Sometimes, it’s the only proposal they feel they have time to read.

Although Thorne is talking about proposals, my bet is that the same rule applies to briefs. Your trial-court motion is likely just one of many scheduled for a particular motion day, and your appeal is just one of many on your three-judge panel’s plate. This means that your brief is just one of many in a big stack that some judge or judge’s law clerk has to read. So put yourself in that reader’s place. Faced with that big pile of paper to plow through, and seeing both short ones and long one in the pile, which ones would you read first?