"We’ve killed a pig." ... "You let the fire go out." Jack checked, vaguely irritated by this irrelevance but too happy to let it worry him. "We can light the fire again. You should have been with us, Ralph. We had a smashing time."
The shell was interesting and pretty and a worthy plaything; but the vivid phantoms of his day-dream still interposed between him and Piggy, who in this context was an irrelevance.
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The sentence should be deleted because it introduces information that is irrelevant to the passage.
The writer should deleted the sentence because it provides background information that is irrelevant to the paragraph.