`BookProof.LorentzGroup.delta_mul_closed` : ∀ x ∈ Delta, ∀ y ∈ Delta, x * y ∈ Delta

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`BookProof.LorentzGroup.delta_mul_closed` : ∀ x ∈ Delta, ∀ y ∈ Delta, x * y ∈ Delta · Prove2Me