⊦ ls(r, a ' + x) = _∃(λ r0. r => a × r0 * ls(r0, x))
ls(r, a ' + []) = _∃(λ r0. r => a × r0 * ls(r0, [])) ⊦ ls(r, a ') = r => a × null
Used terms: r, a, []