⊦ # y0 = # y.tail ↔ # y0 + 1 = # y, y = []
Used simplifier rules: disj-1: ⊦ [] ≠ a + x;%size-02: ⊦ #(a + x) = (# x)+1;% ⊦ m +1 = m + 1; (s)