Suppose we have two symmetry derivations Q 1 and Q 2. Then, [ Q 1, Q 2 ] is also a symmetry derivation. Let us see this explicitly. Let us say Q 1 [ L ] ≈ ≈ ∂ ∂ μ μ f 1 μ μ and Q 2 [ L ] ≈ ≈ ∂ ∂ μ μ f 2 μ μ