and an optional trailer (cleanup code). (“Zero pages” in DSC usually means “one page without the use of the PostScript ‘showpage’ operator.) The functional independence between pages, plus the disallowing of more PostScript operators in the pages section, form the basis for page independence, which allows pages to be reordered, and independently and randomly accessed. This imposed structure is then exposed by delimiting the PostScript file with DSC comments, which normally begin with two percent signs followed by a keyword. Some keywords need to be followed by a colon, an optional space character, and then a series of arguments. Finally, the document is marked as conforming by starting it with a comment starting with “%!PS-Adobe-” followed by the DSC version number. Sections of reusable PostScript code can be modularized into procsets (procedure sets, corresponding to function libraries in other programming languages), in order to ease the generation of PostScript code. Procsets and other PostScript resources (for example, fonts) can be omitted from the PostScript file itself, and externally referenced by a directive-like DSC comment; such external referencing, however, can only work with a document manager that understands such DSC comments. DSC version 3.0 was released on September 25, 1992. The specification states, "Even though the DSC comments are a layer of communication beyond the PostScript language and do not affect the final output, their use is considered to be good PostScript language programming style." Thus, most PostScript-producing programs output DSC-conformant comments along with the code, although some such programs do not actually produce conforming documents. Example A DSC-conforming document (this one generated by dvips) might begin: %!PS-Adobe-2.0 %%Creator: dvips(k) 5.95a Copyright 2005 Radical Eye Software %%Title: texput.dvi %%Pages: 1 %%PageOrder: Ascend %%BoundingBox: 0 0 612 792 %%DocumentPaperSizes: Letter %%EndComments which has the following meaning: marks the document as conforming to version 2.0 of the DSC identifies the PostScript-producing program as dvips 5.95a identifies the document title tells the document manager that the document consists of one page tells the document manager that pages are independent (i.e., not in Special ordering) and appear in ascending order in the document; in this example, since the document only consists of one page, this information is not usually relevant, but will be needed if additional pages are to be inserted by a document manager tells the document manager the coordinates, measured in PostScript points, of the bounding box for all the pages taken together;