Proof 01. 1 ⊢ x ∈ A, hypo. 02. ⊢ x ∈ A → x ∈ A, subj_intro 1. 03. ⊢ ∀x. x ∈ A → x ∈ A, uq_intro 2. incl_refl. ⊢ A ⊆ A, incl_intro 3.