⊦ (o[_∃ xv] * R)[rs, cf, af] ↔ (∃ x. (o[xv(x)] * R)[rs, cf, af])
∃ x. (o[ls(r, x)] * R)[rs, cf, af] ⊦ r = null, r ∈ rs
Used terms: af, cf, rs, R, λ x. ls(r, x), o