⊦ is_arrayref(r, i, st) ↔ (∀ j. 0 ≤ j ∧ j < i → r -- j ' ∈ st)
[[CONCRETESIMPRULES : ([CSIMPSEQ : ⊦ i < 0 ∧ 0 ≤ j → ¬ j < i])]]