s ⊆ s0 ++ a ⊦ a ∈ s, a ∈ s0, s ⊆ s0, s0 ++ a ⊆ s0
Used simplifier rules: sub-ntrans: ⊦ s ⊆ s0 ∧ ¬ s ⊆ s1 → ¬ s0 ⊆ s1;