r ∈ H ⊦ r = null, ∃ H0, o. H = H0 ∪ (∅ ++ r)[r, o] ∧ ¬ r ∈ H0
∃ H0, o. H = H0 ∪ (∅ ++ r)[r, o] ∧ ¬ r ∈ H0 ⊦ r = null, r ∈ H