⊦ (_∃(λ x. (⌜n - # x = 0⌝ ∧ ls(r, x)) ∧ ⌜# x ≤ n⌝ ))(H) ↔ (_∃(λ x. ls(r, x) ∧ ⌜n = # x⌝ ))(H), r = null