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For a proof of the existence of such a set U, see Theorem 3.6.6 of Srivastava's book that can be reached here: https://t.co/8JVC8QxbLQ The idea is smart and beautiful. Define U by (x,y)∈U iff y∈ ∪ₙ V(x(n)) where {V(n): n∈ℕ} is a fixed countable basis fo