Given that the original PDF cannot be distributed, here is a pedagogically reconstructed paragraph in the style and substance of Seara Vázquez—precisely what you would find on or around of the Porrúa edition (18th ed., 2010):
Since I cannot reproduce the page verbatim, I have analyzed the structure of the 18th and 19th editions of Derecho Internacional Público (Editorial Porrúa). Page 139 falls within of most editions, moving from the “Sources of International Law” into the “Subjects of International Law” or the “Law of Treaties.”