Information:  - A prologue or prolog ( Greek  prólogos , from pro , `` before '' and lógos , `` word '' ) is an opening to a story that establishes the setting and gives background details , often some earlier story that ties into the main one , and other miscellaneous information . The Ancient Greek prólogos included the modern meaning of prologue , but was of wider significance , more like the meaning of preface . The importance , therefore , of the prologue in Greek drama was very great ; it sometimes almost took the place of a romance , to which , or to an episode in which , the play itself succeeded . It is believed that the prologue in this form was practically the invention of Euripides , and with him , as has been said , it takes the place of an explanatory first act . This may help to modify the objection which criticism has often brought against the Greek prologue , as an impertinence , a useless growth prefixed to the play , and standing as a barrier between us and our enjoyment of it . The point precisely is that , to an Athenian audience , it was useful and pertinent , as supplying just what they needed to make the succeeding scenes intelligible . But it is difficult to accept the view that Euripides invented the plan of producing a god out of a machine to justify the action of deity upon man , because it is plain that he himself disliked this interference of the supernatural and did not believe in it . He seems , in such a typical prologue as that to the Hippolytus , to be accepting a conventional formula , and employing it , almost perversely , as a medium for his ironic rationalismo .  - A book is a set of written, printed, illustrated, or blank sheets, made of paper, parchment, or other materials, fastened together to hinge at one side, with text and/or images printed in ink. A single sheet within a book is a leaf, and each side of a leaf is a page. A set of text-filled or illustrated pages produced in electronic format is known as an electronic book, or e-book.   - A preface or proem is an introduction to a book or other literary work written by the work's author. An introductory essay written by a different person is a "foreword" and precedes an author's preface. The preface often closes with acknowledgments of those who assisted in the literary work.  - A foreword is a (usually short) piece of writing sometimes placed at the beginning of a book or other piece of literature. Typically written by someone other than the primary author of the work, it often tells of some interaction between the writer of the foreword and the book's primary author or the story the book tells. Later editions of a book sometimes have a new foreword prepended (appearing before an older foreword if there was one), which might explain in what respects that edition differs from previous ones.    Given the information, choose the subject and object entities that have the relation of 'subclass of'.
A:
prologue , literary work