⊦ a ∈ st0 ∪ st1 ↔ a ∈ st0 ∨ a ∈ st1
a ∈ st1 ⊦ a0 ∈ st0[a, st1[a]] ∪ (st1 -- a) ↔ a0 ∈ st0 ∨ a0 ∈ st1, a0 = a, a ∈ st0
Used terms: st1, st0, a0