For the third-order Lovelock gravity, after varying the Lagrangian versus the metric, I found some derivatives of the Riemann tensor which should not be appeared. How can I remove them? Maybe using the Bianchi Identity.

DefManifold[M, 8, {α, β, σ, δ, ι, μ, ο, ς, τ, υ, χ, ω, ν, ρ, γ}]
DefMetric[-1, ℊ[-α, -β], CD,  SymbolOfCovD -> {";", "∇"}]
χ4 = -EulerDensity[CD, 4] // NoScalar

χ6 = -EulerDensity[CD, 6] // NoScalar
ℒ = -21/l^2 + 3 (RicciScalarCD[]) + 3 l^2/20 χ4 + l^4/120 χ6
em1 = VarL[ℊ[α, β],  CD][ℒ // ToRiemann // NoScalar] // 
     ContractCurvature // FullSimplification[] // CollectTensors // CollectConstants

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