(?x)(?y)(?z)(x=y?(z∈x?z∈y))

(?x)(?y)(y?x)

(?x)(?y)(?z)(?u)(u∈z?((u=x)∨(u=y)))

(?x)(?y)(?z)(?u)(z∈y?(z∈u∧u∈x))

(?x)(?y)(?z)(z∈y?(z∈x∧P(z)))

(?x)(?y)(?z)(z∈y ? (?u)(u∈z ∧ u∈x))

(?x)(x≠? → (?y)(y∈x ∧ x∩y=?))

(?x)(?∈x ∧ (?y)(y∈x → (y∪{y}∈x)))

(?x)(?!y)P(x,y) → (?t)(?s)(?u)(u∈s ? (?z)(z∈t ∧ P(z,u)))

源文件鏈接：

https://gitee.com/wu-org/go/blob/research/foo/SetTheory/somethinguseful.md