#(s0 ++ a) = 0 ⊦ s0 = ∅, # s0 = 0
Used simplifier rules: ne: ⊦ ∅ ≠ s ++ a;%size-one-01: ⊦ #(∅ ++ a) = 1;%size-zero: ⊦ # s ≠ 0 → s ≠ ∅;