# x = # x - 1, a ∈ x ⊦
Used simplifier rules: l: ⊦ a ∈ x → #(x -1l a) = (# x)-1;%in-02-01: ⊦ ¬ a ∈ x → x -1l a = x;% ⊦ n -1 = n - 1; (s)