⊦ x = [], x[0, x.first] = x
Used simplifier rules: get-zero: ⊦ x ≠ [] → x[0] = x.first;%l: ⊦ # x = 0 ↔ x = []; from specification list-data%zero: ⊦ 0 < m ↔ m ≠ 0; from specification nat-basic1