`BookProof.ChapterCollapseDiagonal.trace_diagonal_mul` (ρ O : Matrix (Fin n) (Fin n) ℂ) (hρ : IsDiagonal ρ) : (ρ * O).trace = ∑ i, ρ i i * O i i

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`BookProof.ChapterCollapseDiagonal.trace_diagonal_mul` (ρ O : Matrix (Fin n) (Fin n) ℂ) (hρ : IsDiagonal ρ) : (ρ * O).trace = ∑ i, ρ i i * O i i · Prove2Me