Read the passage below. The passage's analogy is qualified. It does not say algorithmic summaries simply is museum labels; instead, it isolates a relation. A museum label selects a few interpretive facts that frame how viewers see an object. An algorithmic summary selects a few textual features that frame how readers understand a document. That relation is used to argue that condensed descriptions guide attention by selecting features rather than neutrally reproducing the whole. What does the author need the museum-label-algorithmic-summary analogy to establish?