σ0 + ⌜x⌝ = σ + ⌜nf0⌝ ⊦ σ0 = σ + ⌜nf0⌝
Used simplifier rules: append-inf: ⊦ ⌜nf⌝ + σ = ⌜nf⌝ ;%append-assoc: ⊦ (σ + σ1) + σ2 = σ + σ1 + σ2;