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

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