The proof of the lemma in https://en.wikipedia.org/wiki/Cantor%27s_first_set_theory_article#Second_theorem https://en.wikipedia.org/wiki/Cantor%27s_first_set_th

The proof of the lemma in https://en.wikipedia.org/wiki/Cantor%27s_first_set_theory_article#Second_theorem https://en.wikipedia.org/wiki/Cantor%27s_first_set_theory_ar... doesn't look right to me. They say "Since the endpoints of (a₁, b₁) are x₁ and x₂", but in the example below a₁ = x₅, b₁ = x₁₂. Am I missing something?