Proof 01. ⊢ A ⊆ B → (∀x. x ∈ A → x ∈ B), bij_eliml incl_equi. 02. 2 ⊢ A ⊆ B, hypo. 03. 2 ⊢ ∀x. x ∈ A → x ∈ B, subj_elim 1 2. 04. 2 ⊢ x ∈ A → x ∈ B, uq_elim 3. incl_elim. ⊢ A ⊆ B → x ∈ A → x ∈ B, subj_intro 4.