Used simplifier rules: eq: ⊦ a ∈ s → s ++ a = s;%size-one-01: ⊦ #(∅ ++ a) = 1;%in-04: ⊦ a ∈ ∅ ++ b ↔ a = b;%Size-insert: ⊦ ¬ a ∈ s → #(s ++ a) = # s + 1;%size-one-01: ⊦ #(∅ ++ a) = 1; Used formulas: b ∈ ∅ ++ a